Multiple reasons: Musk spent a bunch of time bashing twitter's moderation efforts pre purchase. He's then spent a bunch of time mocking advertisers' wish for brand safety [1]. He chased that with calls to harass advertisers who stop advertising on Twitter [2,3].

For context, Twitter has never had a particularly compelling ads product. Like it or not, both FB and Google have super compelling products: the best (by far) tracking, targeting, cross-device linking, or intent. eg Facebook could match your user across devices, browsers, and apps; and allowed ecom targeting by phone or address, not just pixels or cookies. Google has a near-monopoly on the highest-intent ads next to their searches. That's why, in 2021, Google earned $209B from ads, FB $115B from ads, and Twitter < $5B from ads.

Add that up with Musk's ... compelling offer to whip his followers into a rage at you if you, say, reduce ad spend allocation to Twitter in your quarterly ad rebalancing, and it's not hard to see how we got where we are.

[1] https://twitter.com/elonmusk/status/1587899771091566595

[2] https://twitter.com/elonmusk/status/1588676939463946241

[3] https://twitter.com/GoAngelo/status/1588696580709924866