Because React, more so than the other frameworks, puts such a strong emphasis on "the UI output is entirely based on your state", while at the same time there are some limitations to tracking state in ways that are inherently tied to your UI hierarchy.
If you look back at a typical 2011-ish jQuery app, the exact opposite is true.
You might have a modal visible in the page, but the only indication code-wise that the modal existed was in the DOM itself, because there was a one-time mutation of the DOM via `$("#my-modal").dialog()`. The only way any part of the app could _know_ if that modal was being shown was to check on the actual DOM itself, like `$("#my-modal").is(":visible")`. Then you start getting into weird bugs because maybe some other script has mutated the portion of the DOM you cared about, or you're having to write logic to handle if the DOM _was_ in this state and now needs to be in _that_ state.
What we've learned over the last decade or so is that while that is fine for adding small amounts of interactivity to the page, it's not a maintainable way to build large-scale application codebases.
Instead, you want to have an actual `isModalVisible : true` boolean be the real source of truth, and the UI should just be a reflection of that value.
I'm not saying that React invented this idea or that it's exclusive to React, just that React's docs and community has generally put a stronger emphasis on that specific mental approach to designing the app:
What they're saying is that there was no clear separation between model view and controller. Now they've really embraced view (React) as a pure function of model (Redux) that has very clear way that events modify state coming from the controller (your business logic).
