Because he totally mishandled the bid, his communication skills were terrible, it was an internal tech hire (I do not know the process, but was it competitive?), TWTR's board is notoriously dysfunctional (so I am guessing they just went with what Dorsey said, they were quite happy to let him run the company off a cliff...so why not let him appoint his successor), and he appears to have been on leave for a large part of 2021 (when the shit started to make contact with the fan for TWTR).

But he also gave the impression consistently of someone who was in the wrong job. Communication is an important skill for a CEO, the least worrying interpretation is that he was making choices about PR with no reference to his PR people...least worrying...if PR actually was involved, that is worse.