Elon posted this on Twitter (quoted from an private blog since I currently have no way to access twitter):
Temporary emergency measure. We were getting data pillaged so much that it was degrading service for normal users! Several hundred organizations (maybe more) were scraping Twitter data extremely aggressively, to the point where it was affecting the real user experience.
Lets hope that they lift this ban soon.
I'm definitely not using this platform anymore, but Twitter being private also locks away a lot of valuable content (including government announcements, as someone mentioned).