Because from my experience and what I have been told by users of both /pol/ and /leftypol/, people get banned less on /leftypol/ for opposing viewpoints.

I also think that there is a difference between the boards in that /leftypol/ is quick to ridicule a sincere Maoist (Jason Unruhe) but I've seen that /pol/ will jump on the bandwagon of the nearest Holocaust-denier. I think that there is a lot more of a liberal and tolerating view on /leftypol/ due to the sheer diversity of the left. This is in contrast to the very narrow brand of the right-wing (a subsection of the alt-right) that's on /pol/, from what I see.

But we should remember that it's Leftist Politically Incorrect, not Leftist Politics.