To me, the most likely explanation is that Florida Republicans knew that McCain was going to lose anyway, even if he carried Florida, and so decided to save the money to ensure they keep the governorship in 2010 rather than spend it on a lost cause.
Unelectable? Where are you getting this? Obama led McCain in all the polls right up until the RNC. The best that McCain could do at the height of his convention bounce was to pull into a tie, and even that was only in some polls and not others.
Obama was never unelectable. It was just that some people desperately wanted to think he was.