Even so, web developers always had the option of sticking with an imperative approach to UI, like using element.innerHTML = '...', but overwhelmingly they prefe

Even so, web developers always had the option of sticking with an imperative approach to UI, like using element.innerHTML = '...', but overwhelmingly they preferred more declarative solutions like React. You could just as easily make the argument that the shortcomings of HTML had nothing to do with its "declarativeness", since that part was retained. I think a more likely explanation is that HTML couldn't be used to express the more complicated SPAs that people wanted to build, and regardless of what you think about those, JS-based solutions had to fill that need.