They aren't. You're confusing 'left-wing' with 'middle class liberal' and there's a fairly large difference.
Just going from the first few pages of the AV thread:
LOL no. It's not PR and the Liberals shouldn't be rewarded for their poor governmental skills.
No. It's not my preferred alternative to FPTP (which is actually the Scottish system) and I really, really don't like a system that would risk the Lib Dems holding the balance of power on a permanent basis.
I was going to vote no*, but I think I'll take a leaf out of some of the posters in this thread and just spoil my ballot with a statement making clear I wanted PR on there.
etc.