A better question is why on earth a liberal would belong to a virulently far-right party. Though to be fair, nearly all such liberals have finally figured out they don't belong in the GOP. We're still waiting for the yokels in Oklahoma to get a clue.
That's a double-edged question. The Republican Party wouldn't be so "virulently far-right", as you put it, if there were more moderates or even somewhat liberals within the tent.
I think the country's politics were much better when there were more conservative Dems and moderate Republicans to modify partisan polarization.