Latency is far worse with AJAX. Also, browsers nowadays are much more powerful, both in rendering capabilities and Javascript engines. Servers have become a lot cheaper, too, which makes hosting this for free far more viable. Lastly, while a lot of stuff added to JS was just syntactic sugar, the whole ecosystem and tooling improved massively, which makes developing something like this a lot easier.

I'm not saying this was impossible 15 years ago (in fact, Flash-based MMOs existed back then), but it would've taken a lot more effort.