Unity, Diversity and Divisiveness in Marxism.
I have already pointed out in Unity, Diversity and Divisiveness in Anarchism, how anarchism is a good deal less sectarian than Marxism, and how its diversity is both a strength and weakness. The question then arises, "why do Marxists have such a problem with diversity and why is sectarianism such a dominant problem?" I should point out that this has been an historical problem, and that at present, Marxists are becoming a good deal less dogmatic and more prone to working together. (1)
Late 19th Century Marxism was infected with positivism, which took the form of regarding social science as a "hard science". Positivist social science was supposed to have clear-cut, accurate and predictable results, more or less like chemistry or physics. Thus, a political party based upon such an underlying viewpoint could have a "correct line" and every jot and tittle worked out before hand. But social science is not a "hard science". While it is scientific because it uses the scientific method – logical argument and evidence - its ability to predict outcomes is much more general and limited.
A politics based upon such tacit positivism will generate factionalism. Since reality is complex, not simple, a number of "correct lines" will be developed, each describing a facet of that complex reality. Furthermore, the real world will intrude into a party's theory, giving rise to dissatisfaction with the party line and generating a faction. Since there can only be one "correct line" within a party, the faction will end up getting expelled and form its own party around its tiny fragment of reality.
Another factor is the internal make up of most Marxist parties. Internal democracy tends to be based upon representative (bourgeois) democracy, rather than delegated power. There is also no rotation of positions or term limits. This makes it easy for a bureaucracy to develop or a clique to hold power. Thus you have parties with the same leadership for an entire generation. Such rigidity leads to factionalism as some members become dissatisfied with both the process and the politics of the party. The dissatisfaction increases as it becomes obvious that change is well neigh impossible, the group leaves (or is expelled) and a new party is formed. And ironically, 9 times out of 10, its structure and underlying way of being will be no different from its parent body.
For an excellent in-depth analysis of Marxism and sectarianism see