I'm going to guess the reason is that congresspeople and journalists see Twitter as their domain of control now and benefit from the privileges of a blue check mark and the other side being kicked off. Probably Zuckerberg's lack of charisma and physical appearance isn't helping either, however unfair that may be. Another reason might be that the FB newsfeed lacks transparency unlike Twitter which contributes to distrust.
Twitter is where all main stream journalists are. They are not going to bite the hand that "feed" them. Facebook on the other hand has still some amount of non political correctness on their platform so that's something main stream media who wants to control speech online cannot stand.
> I think it's because Twitter is seen as doing more (banning Trump, putting notifications on posts), but is that actually the case relative to what FB is doing?
Twitter certainly has a tighter moderation when it comes to "moderating" alleged conservative speech, yes, which is what matters to the former camp.