Javascript ajax was getting out of hand. Everyone wanted SPAs but managing the state of each button was becoming extremely unwieldy. Thousands of jQuery/js lines, the submit button wouldn't toggle to enabled once disabled...stuff like that. React came along and had prop state per element on a page (as they rebranded it, components), which helped immensely in this department.

Edit: and yes, as you said, it came from Facebook, so it wasn't just a solo developer's grad project; it had a large amount of support right out of the gate.