Zuckerberg had nothing to do with the development of React other than being the CEO of the company that developed it. Jordan Walke created React. React was revolutionary in the day of Dojo and Angular 1 complexity and jQuery spaghetti. It was a small library that let you build declarative UIs. It was simpler and easier to use than most anything at the time. It also brought along with it the virtual dom, which at the time was the fastest way to render an app of any sort of complexity. At the time IE6 was the dominate browser, enterprise or otherwise, and Chrome and Firefox were still the new kids on the block. IE6 did not have JIT compilation and still ran Javascript in purely interpretive mode, so simply by using React made your app 2x as fast.

The complexity, the feature sets, and the move more towards FP-land came later. There is still a pretty straightfoward path to using React rather simply as your UI library.

If you don't get why it became so popular, it just means you haven't had to use much JavaScript to develop apps for at least a decade.