The lack of adoption is a combo of a 1) network "dead-zone", 2) reversal risk, and 3) regulatory friction.
FB & Co are uniquely situated to fix these 3 in a way small companies just can't.
1) Any network is useless unless people you need to use the network are using it. Cryptos are still in the "dead-zone" where there's not enough people using it where it has value as a payment platform. By integrating into existing apps (Venmo, FB Messenger, Paypal) that could change
2) Every fiat transaction has some level of reversal risk. There is legislation in the US that requires, and companies that implement, reversal of payments if the payor claims fraud or account takeover. Any company that sits in between fiat systems and the crypto systems have a huge amount of this risk cause they can't reverse crypto payments. These companies are uniquely capable of handling this risk because they already spending resources to figuring out that you're a real human being and you are who you say you are.
3) Regulatory friction (AML/KYC) is something these companies are uniquely able to tackle because they have your identity. Furthermore, they have the legal resources to find out how far they can push the line and lobbying to keep them from harm when they get too close.