In my opinion, this is because there is no real evidence that socialism and communism can bring about an economy that is satisfying to its populace. No communist country has ever managed to do this. (I do not count China as a communist country any longer, at least in terms of economics.)
Communism seems to work okay for getting a country from economic backwardness to a certain level of development. The Soviet Union, for example, was able to get to a level where it could compete with the West in terms of most military technologies. Even so, however, it was not able to produce the range and quality of consumer goods that its people wanted. It could get them to a certain level, but no farther. That, I believe, is why countries no longer try communism.