Obviously there's no simple answer or single reason, but why go guns blazing to topple a regime Grenada while at the same time going to such lengths to hide involvement in fighting the governments in the other three countries?
IN THE NATION; IS NICARAGUA NEXT?
I think this article from 1983 - the year of the invasion of Grenada - is quite concise and interesting regarding your question.
The invasion of Grenada was seen as ill-advised, and while there was public support for it, the justification of 'protecting medical students' was seen as flimsy. Margaret Thatcher is reported to have complained about the invasion. The U.N condemned it almost unanimously.
Aside from that, Grenada is also extremely small - in the order of 90,000 inhabitants. Nicaragua and El Salvador have about six million. Guatemala, about 17 million. The operational scope would be extremely different, and the amount of casualties sustained magnitudes bigger - a daunting prospect for a country that had just withdrawn from a similar adventure in Vietnam.
The anti-communist counter insurgencies funded and trained by the U.S had all been quite successful up to that point; most Latin American insurgencies had been destroyed, and the continent was firmly in the grasp of the U.S, Cuba aside. Why would anyone risk a direct intervention when destabilization so succesful?