I think a big part of the problem is that liberalism dates back to the 17th century, and western civilisation is kind of built on top of it.
As a result it could fit pretty much anywhere on the political spectrum. I consider myself pretty leftist, but of course I'm a fucking liberal. I take issue with inheritance and I believe in taxing billionaires out of existence, but that's completely consistent with liberalism. And so is disagreeing with me.
I guess a central thing about liberalism is refusing patriarchalism, which would explain why the stalinists and the trumpists alike get upset by it.