They are a poor country ravaged by a tyrannical regime that has lasted as long as it has because Cuba is but a slight annoyance to the US after the Cuban Missile Crisis- nothing more, nothing less. Even if there was no embargo, the Castros would still be there doing as they wish. God knows they're trying to do the same with Venezuela.
EDIT: None of this means I'm pro-embargo, or into US meddling into things they have no business in. Just that ultimately these embargoes do little in the grand scheme of things to stop dictatorial regimes. A tyrant has no reason to play by the rules, especially when money is involved.