The basic idea is if you have n ways your page can display, with the old PHP/CGI whole page refresh model you needed to write n endpoints, but every interaction required a slow and disruptive whole page refresh

With pre-react single page apps, if you had O(n) ways your webpage could display, you needed to write code to account for n^2 ways that each of those n states could transition into another one of those n states

With react, you write code to render n pages, and get the fine grained interactivity for free with good (enough) performance. It basically removed a tradeoff/dilemma that had been facing web developers since time immemorial, so no surprise it was a big hit