Among other reasons, national TV exposure.

Shortly after endorsing John McCain, Gov. Charlie Crist went on Fox News and explained why he took so long to weigh in.

"I was looking this past week who I was going to vote for on next Tuesday and came to the decision in the course of this week and I just felt that it would be the right thing to do to support John publicly. Sen. McCain has been a dear friend for a long time. I think he’s a great leader. He’s a true American patriot. He’s a great conservative and I think he would lead our country well and I just wanted to go ahead and do it. It’s more of a gut thing than anything."

Sean Hannity, who mistakenly called Crist "senator," asked if the governor likes Mitt Romney. "Yeah I do like him. … All these candidates are good candidates. But at the end, you’ve got to vote for somebody."

Hannity reminded Crist about McCain’s perceived weakness among
conservatives. How, then, would he win the base? "Well, you’ve got to
realize what Florida is. Florida is a mega state," Crist replied.
"We’ve got about 2 million veterans … that I think will be an
enormous factor in come this race on Tuesday. I think that’s going to
help Sen. McCain in a significant way …  But in addition he’s for tax
cuts, he’s done well on other conservative issues, he’s pro-life, and
he’s just a great guy. And I think Floridians understand that, they
believe that and they’ll see it."