Over the centuries we have built up a very elaborate currency system. It complex with elaborate protections yet is simple for your average person to use.
It's also very expensive for what it does. Today money is represented by a few bytes on a disk. Transferring it takes a few smallish IP packets and minimal CPU cycles. Ergo, the cost of doing these things should be fractions of a cent. Yet if I buy something using paywave, it costs someone about 1% of the transaction. If I by something over the internet it's more than 1%. If I transfer $1K overseas it costs $20 or something. With costs like that its ripe for disruption.
This system is firmly cemented into our society with lots of law, and this inflow of those huge quantities of cash has created a legion of formidable vested interests who will defend it to the death (most because if it disappears they will too).
This is what Uber was up against in the taxi industry, albeit this is at much larger scale. Uber got around it by just ignoring the law, and in the process bankrupting the species those laws protected - the taxi licence holders. It caused enormous pain for them, and they were most innocent bystanders.
It's going to take a special kind of hubris to take that on, and a enormously talented person to pull it off. Zuckerberg may be that person. I can't say I like his chances, but given he's seen WeChat's success in China I don't doubt his motivation.