The difference is that police officer and firefighter are roles that can be occupied by people of either sex, so the change of language from policeman and fireman made sense in that context.

By contrast, pregnancy is a state of being that is by its very nature female-only. Males cannot become pregnant, as by definition they lack a female reproductive system.

The "pregnant people" terminology implies that pregnancy is something that affects all people, male and female alike. But this is of course not true at all.

It's also not inclusive - consider that much of the sex-based oppression women experience goes back to the ability to conceive children. "Pregnant people" discards and downplays all of that.