You don't throw out a mayor who's got a decent-to-good record. You certainly don't throw him out in favor of an opponent who's got a less-than-compelling message.
He needs to throw the mayor off balance so people will look unfavorably at the mayor. He's got to get under the mayor's skin.
In the transit strike and even before that, he's come across as a feisty fighter, a real mayor of the people.
The mayor took his pain early on. Tax increases were substantial and hurtful, but they hurt early on, and he had time to recover.