I thought what J. Moufawad-Paul wrote about this in Continuity and Rupture was pretty good. I will pull some quotes from it but I'm posting the entire book as well. Read chapter 3: The General Limits of Marxism-Leninism (page 93).
"So what is Clark claiming if he is not simply dismissing Marxism-Leninism as an erroneous politics and advocating a theoretical reinvention of the wheel? I will simplify the above passage to its most salient points.
1. The working-class caught up in trade unions cannot produce a revolutionary organization by itself because, in this context, it is only capable of producing an economism (“trade-union consciousness”), or a defiant anarchism, but not a mediating party that produces a revolutionary movement with a coherent and revolutionary theory. Here we must recall Althusser’s analysis of the philosophy of Marxism-Leninism where he points out that the working-class, which spends most of its time working, can only conceive of its rebellion according to the ruling ideas of the ruling class.
2. And yet the revolutionary party that approaches the working class tends to be a party composed of pettybourgeois intellectuals who are only capable of having a thorough appreciation of theory and revolution because, unlike the workers they claim to represent, they possess a measure of social privilege: they have the time to be students or academics. Here we must recall that, in What Is To Be Done? Lenin agrees with Kautsky’s claim that the party is initiated by the petty-bourgeoisie.
3. The party began by these intellectuals, since it recognizes the proletariat as being the grave-digger of capitalism, must impart revolutionary theory to the workers so that these workers can also be intellectuals. The workers must rely on these intellectuals in order to comprehend revolutionary theory, to understand a revolution that is supposedly about their own interests.
4. A given worker’s intellectual development is decided by the petty-bourgeois educator; it is these petty-bourgeois intellectuals who have the privilege of judging whether or not the workers are learning properly, just as they have had the privilege to decide what these workers should learn in the first place––indeed, what counts as proletarian ideology. Hence the germ of the contradiction: the pettybourgeois class becomes the authority on proletarian ideology when, according to the very ideology they seek to impart, social being should determine social consciousness––how can someone whose class position is petty-bourgeois ever be fully capable of having a proletarian consciousness and thus understanding proletarian ideology?
5. The petty bourgeoisie remains in charge of the movement, its outlook misconceived as proletarian ideology, the meter of revolutionary theory, and thus petty-bourgeois ideology becomes sublimated in the movement itself. Counter-revolution happens precisely because there is an unquestioned petty-bourgeois basis to Marxist-Leninist revolutionary movements.
Finally, we can simplify all five of these salient points, as well as the passage itself, to this basic statement of contradiction: on the one hand it is impossible for the proletariat to spontaneously develop a revolutionary party with a revolutionary ideology; on the other hand it is impossible for a party that the workers cannot possibly develop, and thus is developed instead by the petty bourgeoisie, to carry a revolution to its completion. In essence: Marxism-Leninism is correct while, at the same time, Marxism-Leninism is incorrect."
JMP believes the solution to this problem is in the Maoist conception of the Mass Line and the cultural revolution (he elaborates on that later in the chapter), but I think the above quote is a pretty guess as to why revisionism happens (or at least one of multiple causes), even if you don't agree with him that MLM is the solution.