The problem is that the leftists don't get to learn.
The cause and the inevitable failure are too far apart.

2nd the leftists are usually left out of the quick death scenarios.
They aren't in the group that can actually do anything...

3rd the leftists are in the group that can't do anything.
The person who can perform and rise to the top hates communism and loves the freedom that capitalism brings. The people who can't perform hate competition based systems.

So, given a choice, those pathetic weasels choose communism every time, because in the capitalist system they are forever at the bottom, and in a communist system, they are punished last. Bring everyone down to their level.