Its because they are distorters of marxism, trotskyists include peter hitchens and tony blair; they buy into capitlaist lies about the USSR and AES countries and are imprealist for the most part, thinking that the western way of communism, or the current iteration of socialist projects in AES counteries are bad and not socialist/communist. They most often become neo-liberal conservatives.

While mao-ists can still fall into this trap of believing the lies about the USSR/AES etc, they do often more come at it from a critical angle, the valid criticisms coming from the trot camp are basically non-existant because next to none of it is done in good faith.

edit:

that being said its not like they are the biggest concern of a marxist organizer; you will run into trots while trying to join a marxist org most likely as they tend to have small sects of collective pockets of old grumpy men occupying communist icongraphy and alienating anyone curious in organizing by usually preaching a bunch of nonsense about how cuba is evil or some shit; they usually hold anti-lgbt or suspucious on lgbt issues and again act as a alienating force.

If you are in an org with other trots though its not like these people often cant be convinced around, but it is pertenent to not let them rise to positions of power while still holding onto the falsehoods; its worth trying to actively debate them in meetings and challenge them (as if it comes down to debating on theory grounds they will lose, they dont read and if they do they always fail in interpeting it to real life) but I would say its not worth joining an explicitly trot org. The key give away on the stance is ususually for me, holding anti-china/NK/cuba/vietnam views and publishing them with promince in any publication. It gives away the purpose of these groups, disunity. Same with any anti-LGBT sentiment or if they dont have unconditional support of trans comrades.