Gee, I wonder why they've just now made this decision?

It may be simply because they can. Sure, this may be them knuckling under to the incoming Administration - but it may also be that they no longer have to knuckle under to the last one.

Content moderation on a social media site is a complicated and annoying task. There's a million different things you have to moderate just to meet your legal obligations, and a million more that you have to filter out just to keep your site usable for normies. There's probably a huge desire not to also have to take on the added burden of monitoring everything and making judgment calls on whether things are true or not.

Other things being equal, I wouldn't be shocked if FB just wants to not have to deal with it and be more like a phone company than a publisher - they just provide the platform for connecting, and aren't involved in making sure that people aren't lying during their phone calls. That's not feasible for FB, but I'm sure that their late-stage capitalist preferences were always more towards not spending resources on fact-checking if they could avoid it.