The Three Splits on the Left

There are three splits on the left: 1. The Democrats are split from the Socialists, Greens,Anarchists and others on the far left, and (2) Within the Democratic Party, the progressive Dems and Berniecrats are split from the mainstream, corporate Dems, and (3) Within the far left, the various factions rarely agree and often have weird rules about needing consensus.

Why can’t groups on the left cooperate and horse trade on various issues?