Not because the advertising doesn't work (it may never have).
Not because there's a surge of new propaganda campaigns (anything that is there was already permitted).
Only politics?
When the owner becomes super political then it alienates 50% of the country.
Companies don’t care one iota about politics, they want their publisher to be a partner and a force to propel their commercial product.
Of course they will pull out as soon as they notice that ideology becomes more important than pushing commerce. Which is why what is happening at Twitter. It’s also the reason why advertisers snub Trump’s Truth, whereas they flock to Instagram, Facebook, Tiktok…
I'd assume that, if in a few months, Musk has gone back on his previous claims and Twitter moderation remains much as it was before, they'll return. If not, not.
If he'd announced new policy, and then they headed for the hills, that's different than jumping the moment he owns it. One is about policy, and the other is about politics. They're leaving him, not Twitter.