The main problem is that the state of your frontend application is in the URL. This is not flexible enough for modern UI where you might have many different zones, widgets, popups, etc. that all need their own local navigation, activation states etc. Putting all of this in a single global url is extremely hard. Designing your app so that you don't need to put it all in the global url is harder.
This problem is trivially solved by React / Vue that all provide their version of a state store that can hold the state, and make it easy as well to have elements shared or not between the tabs of your browser.
If you build your applications like phpBB forum this is not a problem, but nowadays users expect better.