It wasn't "doomed to fail" except in retrospect. The regime was tipped off as part of the continuing US policy to make sure that the dictators of the world die of old age.
Even if they had taken out the president, all of his political appointments would still be there. The most likely outcome is being immediately killed by the old military and replaced by some general mostly interested in keeping the status quo.
If they couldn't manage to get enough people, especially locals, for the coup what chance did they have of collecting enough loyal people to rule the country and transition it to Democracy?
Well presumably rounding up support locally whilst still trying to stay undetected is pretty hard. Realistically you'd probably only want to have enough people for the plan to succeed informed, any more and the chance of detection rises.
As it was, the U.S. tipped them off anyway and they were waiting for them, so they probably would have needed a lot of people to succeed.
