Looks like we might finally have the skinny on why Edgar Wright left Ant-Man. It's apparently Edgar who quit, he wasn't fired by Marvel Studios, as some had assumed. LatinoReview is reporting from inside sources that Wright and Cornish we trying their best to appease production notes from Marvel. That didn't seem to be enough and two low credited writers were making massive rewrites to their script. To the point that Wright's vision had been lost, this is according to this article. I guess we'll hear from Edgar soon, but it's unlikely he'll be giving straight answers. Considering he still has a project or two in-development over at Disney.
About 3 months ago, Marvel had notes. The meat of the notes were about the core morality of the piece, must include franchise characters. etc., These notes came from the big four at Marvel. Joe Cornish and Edgar Wright did two drafts to try and answer the notes without compromising their vision.
6 weeks ago Marvel took the script off them and gave the writing assignment to two very low credit writers. One of the writers were from Marvel's in house writing team. Edgar stayed cool, agreed to stay on the project, and read the draft.
The script came in this week and was completely undone. Poorer, homogenized, and not Edgar's vision. Edgar met with Marvel on Friday to formally exit and the announcement went out directly after.
I have my own theory that these "writers", could in fact be Stephen McFeely and Christopher Markus. Since the pair, have had their hands in doing rewrites to "troubled" scripts. Most recently adding their "magic" to Thor: The Dark World, when the script wasn't up to the studios' standards. It would be even more insulting, if Marvel had unknowns reworking the script that Edgar had been writing off-and-on for around eight years.