Not GP, but I would say, it’s the same reason someone would use React. If you keep you state in a single place, the rest of the app can become very functional and pure. You receive data and tranform it (or render it). The actual business logic that manipulate the state can be contained in a single place.
This reduces a lot of accidental complexities. If done well, you only need to care about the programming language and some core libraries. Everything else becomes orthogonal of each other so cost of changes is greatly reduced.
I would imagine the same arguments for Smalltalk like live coding and an IDE within your production application. So you get some overlap with things like Phoenix LiveView, but more smalltalk-y.
I assume it had backend scaling issues, but usually backend scaling is over-stated and over-engineered, meanwhile news sites load 10+ MB of javascript.
Edit - rather than spam with multiple thank you comments, I'll say here to current and potential future repliers: thanks!