> I assume you are talking about geographical distribution of the user base. I don't have experience implementing apps that require heavy geographical distribution, but why does REST/JSON work better for this?

Once you see the smoothness of fully local UI transitions compared to doing a WAN roundtrip, you can't go back, even if it's objectively not a huge difference. Things like switching between tabs, or hierarchical dropdowns where the choices in the second dropdown depend on what was chosen in the first dropdown, doing a network roundtrip to load an HTML fragment is maybe OK over a LAN but it's a nonstarter over the internet.

So your state management needs to be at least partially local to the frontend. And in my experience the only approaches available for that are a) React or b) ad-hoc hacks; some frameworks build the ad-hoc hacks into their builtin components so you get smooth local UI transitions as long as you're sticking to the happy path, but that ends up not being enough.