Overtaken by Events
This is one of those things that happens in the intersection between commerce and reality. When you put together a film that is supposed to be funny and make it about a neighborhood watch, seeing a news story about a horrific killing by a neighborhood watch volunteer harshes the buzz. Couple that with the fact that George Zimmerman is a buffoonish character in and of himself and you're stuck with the task of changing the title. The real-life antics of George Zimmerman are so horrible as to make the subject almost unassailable. We're still waiting for that really great comedy about pretending to be a 9/11 victim in order to get the girl.
Calling it "The Watch" hurts the film. It detracts from the intended choice. It isn't specific enough about what kind of "watch" it's supposed to be about. This is unfortunate. I wonder if Stiller tried to fight it or if he gave in. It could have been that or seeing his film shelved for a year.