At this point, I think some American politicians and policy makers are afraid of Cuba surpassing the US in living conditions if the embargo is dropped, and Cuba is given a fair shot. A thriving Cuba would serve as fodder for a leftward push in politics in the US, and I don't think any of the lobbyists or their puppets want that, at all.

Not to mention that, given the average age of said politicians and policy makers, one can only assume many of their brains are heavily washed with the Red Scare era propaganda.