edit: I know that Stalin having Trotsky assassinated played a part, but the conflict between authoritarian and anarchist communists predates the assassination, and that also doesn't fully answer why the conflict is so persistent.
It's difficult to incur what it is that is bothering you from the way you have worded your question because Trotsky and all people in the Fourth International would insist on being Marxists-Leninists themselves.
The Fourth International is reviled by a plenty of people for a very large variety of reasons.