I think there are two main reasons.
(1) It's relatively easy for non-leftists to unite, because they dwell ideologically in the past and present. There is just one past and just one present. They can say simply, "name something that's unfair and demand that it be stopped" (liberal) or "dont change anything at all" (conservative) or "turn back the hands of the clock" (reactionary) - then - voilĂ*! - instant unification. However, the left is about the future being detached from the past, the future being redesigned intelligently. There are an infinite number of possible futures, and therefore an infinite number of possible sects that we could form.
(2) People on the left know intuitively that some goals and strategies are workable and some are unworkable. But there is disagreement about which goals and strategies are the workable and the unworkable ones. Do things the wrong way and there may be defeat. There may be no basic social change at all, perhaps, the reform band-aid distraction, the perpetual two-party flip-flop, popular disenchantment with any change, etc. Or there may be what appears to be a revolution but which moves toward a new dystopia. Therefore, we have the desperate feeling that it matters very much to get on the right path, but without an infallable way to determine which path is the right one. We sense that it's a matter of great urgency to leave port and launch the ship, but without clarity about which harbor we are supposed to be sailing for.