Because you would have to add runtimes for all those languages with ties to the DOM. It's hard enough to keep JavaScript secure. It would be a nightmare to keep different implementations of JavaScript, Ruby, Python and client-side PHP secure on different operating systems.
The way I see it, web-apps today are downloading javascript code that calls back to the server from time to time and outputs a DOM tree. Any scripting language can do that, right? Why can't I write my client-side code in Ruby, Python or PHP?