For the same reason why government don't like to use the term propaganda, it's pejorative. Technically propaganda is just "information to promote a cause or point of view", but in the modern world it doesn't mean that. It means lies to control you and is associated with dictatorships.

Sure. Cult means what you said it does. But when people use that they hear (and usually mean) not something that's strange, but what is basically a con which sole purpose is to exploit vulnerable people, steal their money, cut them from mainstream society and usually does illegal stuff. A cult is dangerous and should be avoided.

Now I can already see the answers, "that sounds exactly like religion". Don't get me wrong: there are absolutely established a lot of mainstream religious organizations with cultish behavior that should be stopped. There are pastors who are merely conmen, televangelist usually are borderline cult leaders (and some actually are that). But the majority of churches are not that. Your average synagogue is not a long time con to exploit local jews, your average church is not telling faithful to "never interract with someone not in this room because the government that's the root of all evil will kidnap and torture you!". It's just not the case.

So religious people don't like when you call their religion a cult, because you are saying to them "you're being exploited by a conman that want to harms you".

It's the reason why modern studies of religions don't like to use the term cult, it's just too pejorative.