Similar situation, but different enough.
Grenada was facing a commie takeover, sponsored by Cuber and of course slightly, sort of directly by the Soviet Union. Another Cuba, this time in the southern Caribbean was not in our interests.
Grenada is not exactly in our backyard but certainly just over the fence. Monroe doctrine anyone?
As opposed to Grenada, the political affiliation of the tyrant of Libya is irrelevant to us.