Too many combinations of operating system, cpu, browser, browser version -- the server would need to have a huge number of versions built and cached. And then y

Too many combinations of operating system, cpu, browser, browser version -- the server would need to have a huge number of versions built and cached. And then you need to ensure your server continually has support for all the latest browsers. But even that pales in comparison to the biggest reason, which is security. In order to run fast, the compiled version needs to be "trusted" to do the right thing; by allowing the server to send pre-compiled code, you're opening up a huge number of attack vectors.