Why did they hire Alessia Cara to sing the leadsong? She also sings it in the movie right (haven't seen it yet). Why don't they just use her also as official leadsinger? Do they think she can't pull it off?
One for the movie with the voice actress, one for the radio with a current popular singer. In English anyway, in French, the voice actress does both.
Reason: Advertisment, and to make it more radio-friendly to general audience. It's easier to sell them a song by a relatively well-known singer than by somebody new or a bit underground.
I like 100 times better the version of Auli'i Cravalho, but I get the idea behind the marketing. I like the pop version of "Let It go" in French though (but it's the same singer, so it helps).
It's actually nothing new, really. I just watched recently "The Hunchback of Notre-Dame", and same here, they got Bette Midler for "God helps the outcasts" and they got Celine Dion for "Beauty and the Beast". So this a no new trend, maybe we just started paying more attention?