The brand Twitter had a lot of negative reputation due to its previous moderation policies, Rebranding it as X allowed for a clean state and a new focus as the platform of free speech. It's still not fully rebranded, I still see the twitter domain come up when linking tweets which might signify it's not a permanent rebrand. Also people will still refer it as Twitter or Twitter/X, kind of when MSN Messenger had rebranded to Windows Live messenger, people still referred to it as simply 'MSN' because it was easier to say, and the name Twitter is a lot more user friendly appeal than just X.