* focusing on one language across the several browser vendors reduces the resources required for the different dimensions that each language supports : security, compatibility, performance, communicaton overhead for language standardization
* LSP does not define execution semantics, it defines text markup. A browser need to execute code, not just show it. So, now, a given browser needs to have a language plugin or call out to system programs to handle language X. That was a big hassle when they used to do so with the jvm and flash because of the same issues as above: is the version compatible, is the communication secure, do the different language vendors and browser vendors communicate their changes to each other in a timely and efficient fashion ?
There are a couple of responses to your question:
* focusing on one language across the several browser vendors reduces the resources required for the different dimensions that each language supports : security, compatibility, performance, communicaton overhead for language standardization
* LSP does not define execution semantics, it defines text markup. A browser need to execute code, not just show it. So, now, a given browser needs to have a language plugin or call out to system programs to handle language X. That was a big hassle when they used to do so with the jvm and flash because of the same issues as above: is the version compatible, is the communication secure, do the different language vendors and browser vendors communicate their changes to each other in a timely and efficient fashion ?
Network effects. This is fundamentally the same as asking why people dissatisfied with the major social media sites don't just start their own.