WebSockets use Cubic congestion control - this is an option that cannot be changed and is baked into your operating system. WebTransport lets you use BBR congestion control … which is substantially better for applications that demand low latency networking. How tf is WebTransport not yet standardized despite being in development for 5+ years now?
The answer is that Safari is holding back. It’s an age old tale of Apple stifling innovation bc they want to keep their App Store monopoly. How many multiplayer games out there are deciding not develop for web platform and instead iOS because the realtime networking options for web are so shitty? WebSockets are 15+ year old technology and not up to par with the networking needs of modern realtime applications.