Because Pinochet's coup led to freedom and prosperity and the elimination of every Commie he could find. Not enough unfortunately.
Allende's version of governance would have led to Che Guevara on every wall.
Got it?
The US may have succeeded in eliminating Allende with the 1973 coup (a unnecessarily messy way of removing an unpopular democratically-elected leader) but by doing so it succeeded in making Allende a popular hero across Latin America. And now what? Today, I think as a direct consequence, we're not talking about one vaguely socialist government, we're talking about Venezuela, Brazil, Ecuador, Chile, Peru, Argentina and Bolivia. It makes no sense to win a country but lose a continent.