Answer:

A leftist can't win an economic argument against the right, because central planning fails. But, the leftist CAN win the moral argument against the right in terms of personal liberty, rights, tolerance, etc. So when the damn libertarians come around and pull that rug out from under the leftists also, they get understandably pissed.

So a leftist can't win economic NOR personal liberty arguments with a libertarian. That is why they hate them - and would much rather debate a conservative. Leftists and rightists balance each other out... they are both half wrong on the issues and thus can perpetuate arguments indefinitely against each other.

Libertarians make the leftists look like inconsistent fascists (which they are), and they also make rightists look like inconsistent socialists (which they are). It's no wonder that everyone hates a libertarian. His consistency is confounding and infuriating.

The last defense of the left, and the right?  "Libertarianism can never work," they say, and so they go back to arguing amongst each other to figure out who is more effective at enslaving humanity.