He says the time just feels right.
Daley is getting out while the getting is good. Two factors are contributing to this.
1) The feds are closing in. Rod Blago, our former governor, is the first target of AG Patrick Fitzgerald to not get potted at trial. Admittedly, it's a mistrial, but the AG's are already adapting, dropping charges against Blago's brother, forcing his team to rethink their strategy and without a distraction to confuse the jury. Blago goes down, a LOT of secrets become leverage to get his sentence cut and keep his cult of personality going. Blago is too much of an attention whore, and given his daddy is Dick Mell from the old Chicago machine and all the deals he cut, a lot of secrets are at risk here.
2) The Chicago budget. Daley has been generous with the city contracts to his connected buddies. When the recession hit, Daley found his cash flow from tourism getting choked off. With all the funds tied up with contractors, there is no money to do things like hire more cops. In fact, gangs in Chicago have become more brazen of late, recently killing a Chicago chief in full uniform as he was cleaning his car in his driveway. The only way to get additional funds will be to raise taxes, which everyone is already pissed about, and without them, Chicago is going to become a rough town. Daley doesn't want that happening on his watch, so he's running for the hills.
Naturally, attention is now focusing on who will get it. Rahm Emmanuel, the White House chief of staff, was the head of Daley's Hispanic patronage army and has stated he wanted to run for mayor. Daley would have shut him up if he was going to run again, so Emmanuel tipped his hand. Emmanuel has connections, which will also make him a ripe target for muckrackers looking to put an R in charge of the city for a change. If the machine wants to keep control of the city (control of the state is in serious jeopardy), they'll need everything they got. Mell and Dick Madigan are going to do whatever it takes, and that means finding out what kind of support Emmanuel has. If he can't deliver a clear win, he'll be roadkill as they find their own king to appoint.
Whoever takes over as mayor has a huge mess to clean up, and few tools to do it with. I do not envy them, and if it's a good person, I so wish them luck. They'll need all they can get.