If the German government were to make an official announcement that they are switching to https://social.bund.de/ on the television, most of the interested German people would open an account overnight.
Agreed. If someone sends me a twitter link, even if public, I can't read it as my privacy settings are such that they can't get a unique ID on me. It forces a log in. Which I can't, and won't do.
My colleague can view public tweets, even if not logged in, as we assume Twitter can uniquely identify them, even if not logged in.
They've stopped forcing a log in entirely, usually you can view the post that's linked just not any subsequent posts (which of course still ruins post threads, where the user spreads a single post over multiple entries).
My guess would be because of the reach? Like, Twitter has a lot of users and a lot of important figures in politics worldwide use it. Not that many people, both as in political figures, and small users use the Fediverse.
I use neither Mastodon nor Twitter (I've never understood the value of microblogging) but one of them is freely available, the other one is restricted. In my view – as someone who has no account on either platform and only occasionally goes there if it's linked somewhere – Mastodon has a much wider reach.