The first one is that Elon Musk just paid a high price for Twitter at $44B.
Elon Musk wants to have a free hand to shape Twitter the way he wants to make it the champion of the online free speech he dreams of.
The second reason is that Elon Musk might be a bit vindictive ... After all, Twitter CEO Parag Agrawal gave him a hard time when Elon Musk was trying to cancel his takeover offer. He did his best to protect Twitter's and shareholders' interests as his mandate required, but that was tantamount to making life difficult for Musk.
Musk must see this as an opportunity to exact a little revenge.
In any case, it is very likely that Parag Agrawal did not share the same views as Elon Musk for the future of Twitter. His departure was therefore inevitable in any case. So the sooner the better. At least no one is wasting their time!
