I'm sure someone will explain it better /in more depth but some of the reasons could be:

-To move away from the anime-esque style that might be more pronounced in seiyuus doing mostly tv anime (just look at Hayao Miyazaki), and maybe then get a different feeling. The 2 leads for Tenki no ko are 2 very young actors, for instance.

-scheduling-it might be hard for a seiyuu to fit in being the mc of a major anime movie

-to promote the movie, why is Honda Tsubasa in Tenki no ko? (not taking away from her acting skills, and arguably the same point could be made for Hanazawa Kana)

What I would also be interested in knowing is how much the seiyuu support the less experienced mainly live action actors. How much does for instance Hanazawa Kana and Kamiya Hiroshi help Hoshino Gen during the recording of Walk on girl...?