The fan-favorite director, the guy who shot a whole test reel that fans and execs alike loved, the guy who actually wanted to make a movie about a hero most directors have probably never heard of...they got rid of that guy?
Yep. "Differences in their vision" is the reason given. And they presumably don't mean Paul Bettany.
Here's my educated guess: Edgar Wright wanted to make Ant-Man as a quirky standalone film in his own style. Back when Kevin Feige's plan was to open the archives of minor characters and let filmmakers "play in our sandbox" (as he put it the one time I actually visited Marvel studios, during the making of Iron Man 2), it was cool for them to do whatever they wanted. Now, it seems like Marvel doesn't want to just let minor characters do their own thing - every film has to connect as part of a grand "Phase" scheme. If I had to bet, I'd reckon that's where the conflict happened.