Because it's more of a "let's teach a language properly". A lot of people wouldn't accept "gonna" as a valid translation - I'm one of them. Many non-native speakers do the reverse tree. I have taught many of my students to never say "gonna", "wanna", "gotta" (although I am a bit more lenient on "gotta" due to the difficulty of separating the two words). I'm glad DL doesn't accept it. The English language is being murdered by Facebook, let's not allow it to be murdered on a language learning site.