Because it was a massive money pit that was already failing. If he wanted to have a microblogging site at x.com he could have built one at x.com and paid everyone interesting to use it for a year, for far less than he paid for Twitter.
They sold it for a lot more than it was actually worth, got actual cash from it and got a return on their investment, why wouldn't they have sold? Twitter was barely profitable when he bought it, not selling would've been crazy.
We heard this story a million times, but who's to say what's Twitter really worth? Musk bought much more than a "barely profitable website" that day. And the liberal leaders should have seen this coming. Yet they were busy laughing out loud, and mocking, and making predictions on how he'll run the site into the ground, and humiliate himself, etc.