I was about to search why not mali ... I found why here.
Duolingo is funny to use, but there is such a lack of informations... We have to have a look herein the forums to try to understand (if by any chance we found a good guy giving good explanations), then go further on internet. Nice introduction, nice starter to a language... but how many times I have to stop and think : " hey, but why this and not his ? etc... " But maybe it's also the purpose of Duolingo, make us search further. In my opinion there should be some kind of pop up windows (on demand) to explain the general rules of the grammar.