Dwelling on official forms of a language is not a very scientific approach. Official languages are actually quite harmful to the health of a language and its community of speakers. That said, I learned Hochdeutsch (standard german) in high school and though I had an excellent teacher it did not prepare me for the spoken german I would find travelling in europe or the varieties of german I would encounter reading german literature... One of the issues duolingo has in its courses is the problem it has with linguistic variation. Certainly true of courses where I had a strong background in the language already... eg Irish and Dutch. The more prescriptive duolingo is its approach the less credibility it will have for language teachers.
To learn slang, you need first to learn standard language, especially because A1-B1 speakers need to know how the language works and to survive in real life situations (restaurant, shops etc.) - slang isn't important on that stage of learning, it even may confuse poor learners.
I would love Duolingo to, in later chapters, teach us slang though - I really miss those things, as I want to sound as natural as I possibly can. As well as I really miss courses to teach swear words, as they are important part of almost every language and are used almost daily.
I understand your point of view but I disagree. If Duolingo begins to accept colloquial forms, which forms will be accepted and which not? That's why it is better to accept only the "official" language forms. Depending on the speaker's dialect sinulla could be at least sulla, sul, siulla, siul, snuulla...
All Finns are taught to write "sinä" and "sinulla", so people are used to using these when writing, but of course in more informal writing people do tend to use the variants they'd use in spoken language. The course is still in beta and there are many other issues that need fixing, so it'd be difficult to start accepting all possible variants people could think of as there are quite many of those. :)