Looks like Edgar Wright left Ant-Man due to conflicting differences with Marvel over their visions and direction for the film. Whilst its a huge shame I am still looking forward to seeing what Marvel and the future director will do with the story.
Taken from superherohype.com:
The internet community was collectively shocked this past Friday when it was revealed that director Edgar Wright had parted ways with Marvel Studios and would no longer be directing the big screen adaptation of Ant-Man, a film he had been attached to since 2006. In a joint statement, the pair said the split was due to “differences in their vision of the film.”
The next day Latino-Review brought word that the reason for Wright’s departure was the script for the film. Marvel Studios reportedly had some notes about the script and tasked the rewrite to someone other than Wright and his co-writer Joe Cornish which resulted in a script very unlike Wright’s vision for the film. Now, The Hollywood Reporter is echoing this story in a new piece but also elaborates on a few other behind-the-scenes problems, including the heads of the departments for the film leaving the project a few weeks ago when it became clear there would be delays in the start of production.