I think there's just such a huge opportunity to drive sales through messenger, fb, etc with low-friction payments. They already have the 'best' targeted advertising. They just want to extend their platform all the way through to actual sales. Then they can justify a larger cut than with just the ads.
They've certainly tried payments and money-transfers before. This isn't entirely new territory. It's just very country-specific. They get hit by high transaction fees. No one wants to type their full credit card and address info, then deal with required two-factor authentication in Europe, etc. And nothing is worse than having to forward someone from your app to an unreliable slow desktop-targeted bank page which then sends you an SMS/email that you have to type back into the bank.. etc.
FB is actually right. The credit card system is straight broken from a user-experience point of view. It wasn't created with computers, mobile, or basic security in mind. It's just band-aid on top of band-aids since the 70s. And obviously cash doesn't have much presence online in western countries. FB struggles to get people to buy things through their apps. They desperately want to fix that.
Crypto currencies just work better at a fundamental level with mobile payments. It has always just been a lack of institutional support to get them supported. VISA/MC have probably been looking for a way to leapfrog bitcoin/etherium for some time now. I'm not surprised they jumped on board when a bunch of real technology companies decided to leapfrog them. Otherwise, they'd likely be in for a world of hurt in a few years if something like Apple/Google Credit launched their own without them. It's not like people love VISA/MC as companies. A huge number of people would jump ship so fast if there was literally any other option.