Perhaps you're confused, but the rest of Europe isn't confused at how Twitter has a lot of power:
German Chancellor Angela Merkel objected to the decisions [of banning Trump], saying on Monday that lawmakers should set the rules governing free speech and not private technology companies.
https://www.bloomberg.com/news/articles/2021-01-11/merkel-se...
I don't think you understand free speech in the US context, Twitter is protected by the first amendment, it itself has freedom of speech and of association. It does not have to allow anything it doesn't want on its platform. It could ban everyone wearing blue in their profile photo, it is THEIR platform. If Twitter COULDN'T ban anyone they wanted under US law THEN it would be a violation of free speech.
And that wouldn’t be an issue if twitter wasn’t so dominant for this form of communication between positions and people. It’s because it is so entrenched without a meaningful equivalent service that this becomes significant.
It’s hard to draw clear lines between domains, but I think for public representation of official organisations, there isn’t any other. So for this purpose it’s dominant.