Why did Matt Damon opt out of the Bourne Legacy?

To quote from an interview Damon gave:

I think in terms of another one, the story of this guy's search for his identity is over, because he's got all the answers, so there's no way we can trot out the same character, and so much of what makes him interesting is that internal struggle that was happening for him, am I a good guy, am I a bad guy, what is the secret behind my identity, what am I blocking out, why am I remembering these disturbing images? So all of that internal propulsive mechanism that drives the character is not there, so if there was to be another one then it would have to be a complete reconfiguration, you know, where do you go from there?

To summarise, he simply felt that the role was exhausted and it made no sense to have another film with his character - there was nothing left to learn or reveal about Jason Bourne, so he chose to walk away.

He gave a different interview, to GQ Magazine, where he said:

“If you look at the first three movies, we kind of pounded that idea of identity and amnesia into the ground. We really got everything out of it that we could. So to reboot it, we need to come up with something completely new.”

This supports his comments in the first interview, but also shows that he wasn't against a fourth film - he just didn't want it to involve his character.

Of course, that was then. Things change.