Perhaps it's another Ego thing....
JFK is the Democrat's ALL TIME HERO, and to remove the embargo would be to admit defeat, and then somehow sully his name.
Or perhaps we just hate Castro and want to wait until he croaks, and the old bastard just continues to hang on.
I think it's the latter, personally, and as soon as the Old SOB kicks the Ash Can, the Embargo will be lifted within 12 months, "in the spirit of pomoting democracy to a new regime".