In my opinion, having two syllables isn't necessary for a number between 0 and 9. It just makes things more complicated. Wouldn't it be better if the word was simply "un" so the words for "first" would have the stress on the same part of the word as the number (i.e. why unu but unua rather than just un and una)
I'm not sure who voted this thread down or how many downvotes you'll get (update: it was at -1 when I saw it but is now at +1). It seems like a legitimate question to me. The problem, though, is that in the end Esperanto is an established language and your opinion doesn't matter.
If your intention was to expand your knowledge by trying to understand the reasoning behind the original selection of "unu" (130 years ago), this could have been made more clear and it probably would have gotten a better reaction.
In fact, there may be several reasons for selecting the word unu instead of "un" -- and one of them is that it makes it possible to use the form unuj.
Probably got downvoted because of the opening lines that, on first reading, seem to suggest that we are all a bit silly for following the 130 year old rules and should alter them because the poster prefers one syllable words for numbers. It's kind of up there with Japanese should change its scrip becasue i don't like kanji or german should only have one word for 'the' - possibly true but not really how languages work. I guess that is the problem with written communication over the internet instead of a fun discussion over some snacks and a few beers ( if you are old enough ;-) ) that this would have been in real life.
A number of beginners are understandably confused at first and think this constructed language is an under-construction language. Zamenhof spent 10 years creating, testing, and perfecting his language, by translating literature and testing his language. He translated the OT, for example, and tested it on friends. I've thought of theoretical improvements and discarded them all because it turned out that Zamenhof had a better way after all. There is a reason why most created languages after 1890 resemble Esperanto or consciously take it as their departure point. (Exception: movie languages)
Zamenhof began with a lot of cases and whittled them down to only two, what Wikipedia accurately calls the nominative-oblique and the accusative-allative, which are initially hard for people whose languages have rigid word order, but they free up word order and give many prepositions painless double duty. (en la domo, en la domon)
He reduced the burden of vocabulary learning by borrowing the German method of prefixes, suffixes, and agglutination (Enhavo = Inhalt, Malsanulejo = Krankenhaus), and invented the prefix mal-. As he experimented, he improved the system. He dropped a lot of suffixes to keep their number down.
He reduced confusion by changing words. Planets and cigarettes are not little things, so they are planedoj and cigaredoj to preserve the meaning of -et, which comes from French. Receive is not "ceiving again" and report is not "porting again," so he made them into ricevi and raporto to preserve the meaning of re-, which comes from Latin. When we borrow words into Esperanto without changing initial re- to ra- or ri-, we distort the genius of the language.
The article la has the same ending as an adjective and kaj borrowed from Greek looks like a plural adjective—they fit in quite well. Slavic languages don't have articles, so he omitted the more difficult article, the indefinite one. German has a negative article; he didn't include it at all. Romance languages have a definite article beginning in L, Germanic languages have a definite article beginning in D (TH in English), but TH is wisely not in his phonology, and D would collide with Latin prepositions.
He took prepositions from Latin because in his day, everyone learned Latin in school and would know them right off. He changed "in" to "en" to differentiate it from the suffix, and borrowed je from German. He took Esperanto "ju… des" from "je… desto"
He took words in a given category mainly from the same language, such as prepositions from Latin and time terms from German (Tag=tago, Monat=monato, Jahr=jaro, Jahrzehnt=jardeko, Jahrhundert=jarcento, Jahrtausend=jarmilo), The word "morgen" means both tomorrow and morning in German, but more often it means tomorrow; he differentiates them with morgaŭ and mateno.
He made three kinds of adverbs: adverbs derived from other words -e, underived adverbs -aŭ, plus a few monosyllabic adverbs from German such as nur and nun.
Esperanto is a masterpiece, well thought out. We spoil it when we introduce things like the Romance word hospitalo instead of the native Esperanto word malsanulejo, which is more international, comprehensible to people of Romance background, but follows the pattern in German, Dutch, Finnish, Swedish, and Hungarian, among others.
Most people who want to improve Esperanto, "improve" it by making it more like their own language, by cutting and sewing the fabric, creating seams that can tear. To my mind, "kamarao" is a lazy way of saying "fotilo" that impoverishes us, because then we can't distinguish a fotilo from a filmilo.
Zamenhof created the only constructed language in history to become an actual living language, small but widespread, with more speakers—actual speakers—than Icelandic and almost as many as Finnish. Esperanto is so good that even though it did not meet the goal he designed it for, it found its own goals and surpassed them. It can express any human concept and can be applied to any human task. It unites the world, not on the diplomatic level, which was his aspiration, but on the grass-roots level, which was his inspiration; it is the language of people not nations. It is the language of websites, videos, and music. Esperanto has succeeded beyond his wildest dreams, in ways he could not have imagined and in ways we often do not see.
Imagine sitting Zamenhof down in front of a computer. What would astonish him more than the computer, the Esperanto Wikipedia, the Esperanto music video from France, Evildea from Australia, the Esperanto stand-up comic from Spain, or the Esperanto congress in Korea?
I tried to suggest that it was voted down for expressing an "opinion" about what Esperanto should be like, rather than asking a question about how to learn Esperanto. It's a very common reaction among people new to Esperanto, and I would hope that the users of this forum could learn to be more mindful how off-putting and unwelcoming a downvote can be to a new user - especially in a case like this where it's really a legitimate question.
I also think it would be nice if the original poster would come back and comment... even an "oh I see" or "thanks" would be nice.
While we're on the topic: We have one, two, three, four, five, six, eight, nine, and ten, so why two syllables for seven? Why not change it to "sev" or "sen"? Or why the outrageous luxury of three (count them, three!) syllables in "eleven." Can't we start saying "len" instead?
Same answer as with Esperanto, that's just the way the language is, other people speak the language, and we want them to understand us.
Though I believe that "un" is officially acceptable in rapid counting. "Sev" is not. Correct me if I'm wrong.
I am beginning to think that Zamenhof choice "unu" because it is hard to mistake it for an indefinite article. (There are no articles in Balto-Slavic languages. A definite article is hard, and an indefinite article is really superfluous—as we can see when we speak Esperanto. We get along fine without one.
That may well be so, but I'd like to know on what basis you're beginning to think that. To my knowledge, Zamenhof never said that this was the reason. In contrast, he did say that he wanted the word to end in U because he wanted to be able to form the word unuj.