The web browser is both a critical UI technology - it's the only working truly cross platform sandbox - and therefore a battlefield between potential monopolists that has stagnated like a WW1 field of trenches and wire.
Because only the tiniest, most incremental changes can be made to the browsers, and because there are fundamental native/universality conflicts all the time, making tiny improvements to UX requires huge behind the scenes troop movements. And then it's subjective, so maybe it just makes the UX worse in the end.
UI frameworks seem to be surprisingly hard conceptually. Neither Microsoft nor Apple have really proven that they can do better when on a field entirely under their control.
Not to mention business model considerations, such as the worsening of Reddit.