Why not ss instead of ß?
They are both valid spellings.
Always using "ss" instead of "ß" is not 100% correct. There's a simple rule that tells you when to use which. It's "ss" after short vowels (muss, Russland, dass, Klasse, Kuss, Schloss) and "ß" after long vowels and diphthongs (Maß, Fuß, Kloß, reißen, groß, schließlich, außer). In other words, the distinction is actually quite useful in terms of pronunciation because it lets you know whether the preceding vowel is short or long. In Switzerland and Liechtenstein, they got rid of the letter "ß" and they always use "ss" instead. Also, it's generally accepted in all German-speaking countries to use "ss" instead of "ß" if you can't type German letters on your keyboard. Having said that, I highly recommend you do use "ß" as it will help you get your pronunciation right.
@shu: Just to clarify, "Hochdeutsch" (standard German) is not limited to Germany. The Swiss use "Hochdeutsch" as well, even though they tend to call it "Schriftdeutsch". As the name suggests, its use is largely limited to the written language. The local Alemannic dialects are a whole different story. http://en.wikipedia.org/wiki/Swiss_Standard_German
Glenda: but Duolingo is teaching Hochdeutsch, not Schweizerdeutsch -- the pronunciation and the vocabulary are both very indicative of the former, and it's technically wrong spelling to use ss in place of ß in words like weiß in Hochdeutsch.