This is because, as Marc Forster told ET, Jim Cummings has lived with this character for 30 years, voicing Pooh in the animated movies, The New Adventures of Winnie The Pooh TV series, the 2011 film and even the Kingdom Hearts video games.
The nostalgia and the emotion that comes with it only works with Jim Cummings as Winnie the Pooh.
It is a good thing that having Jim Cummings voice Winnie the Pooh was important to Marc Forster, because I shudder to think of the alternative.
Marc Forster also said that this will be a very emotional movie (you don’t say).