Facebook is 233x larger, profitable, and self-hosting - they don't have to keep their cloud service provider happy.
Parler was dependent on AWS for services but didn't comply with their terms.
Facebook is bigger, and has power.
That means that (a) they're harder to attack, and (b) they have purposes beyond ONLY fomenting insurrection and genocide. That means they'll operate in more of a gray area by intention, and will in fact try to self-moderate when the cost of fomenting insurrection and genocide risks becoming more expensive than the profit (ideological OR financial) for fomenting insurrection and genocide.
The difference between Parler and Facebook is, Facebook will take your money but then throw you under the bus if you're malicious and endangering their platform. Parler went down with the ship because it was more important to them than just taking the money.
I don't understand why they should have expected everyone to deplatform them. Facebook hosts the same content in far greater quantity and has clearly demonstrated that they can't moderate effectively, but there's no bulls-eye on them over that. What's different about Parler besides the ownership?