chronovore: Conventional wisdom is that if you want to be president, you need Florida, and if you want Florida you need at least a big chunk of the Cuban voters.
The larger point is that there's a very active, organized, emotionally invested interest group for keeping the embargo, no significant anti-embargo interest group to counterbalance, and an overwhelming majority of politicians and government officials who don't care much one way or the other, but who recognize what the easy choices are.
It's basically the same with Israel, but saying so gets you blasted as an anti-semite on the cover of The New Republic.