I can think of a few reasons, in order from most important to (perhaps) least:
Originally Posted by DisgruntledGoat
1. Security: Right now, he can get paid the big bucks - and get paid those big bucks for many years. But if he has a poor 2013 season, or if he suffers a nasty injury, then there's no guarantee that he will make anything like what he is being offered now. So an extension would offer a lot of financial security.
2. Comfort: He's been in New York for his entire career. He's familiar with the ballpark, with the city and the region, the media and he has friends in the locker room.
3. Legacy: An extension takes him through most if not all of the remainder of his career. He is "the" guy here, and will remain so if he stays - like Jeter. Even though, due to age, there are now better players on the Yankees than Jeter, he is still "the" guy. So getting locked up here secures his legacy and his big man on campus status.
4. Fandom: Wright has been a Mets fan since he was a kid. Professionals don't make as much of a big deal about this as we do, but I'm sure it's a factor.
Depending on how important winning is to him, he's going to have to consider whether he trusts or doesn't trust Sandy's plan. R.A. Dickey surprisingly came out and said he has "bought in" to Sandy's plan. I'm stunned about that, but I think if Sandy can convince Dickey, then it should be even easier to convince David.
Sandy is a proven liar and a shill for the worst ownership in baseball