He bought it because he couldn't stop being an arrogant edgelord keyboard shit-talker, and he accidentally made himself personally legally liable doing so. The Twitter board called his bluff, seeing the most amazing opportunity to rid themselves of an expensive and slowly dying company, and Musk had already pissed off the SEC (or one of the other financial government services) enough that he was at risk of having his finances in other companies audited.
So he was forced to buy it because he's an impulsive troll. Everything else since then is pretty incidental, and unsurprisingly dripping with hypocrisy like this.