Why can't Python? Why can't Ruby? Why can't Perl?

Fundamentally, the languages able to run in the browser are only limited by what the browsers want to implement. It would be great if browsers could render QML, or could use <script type="text/python"> or <script type="text/scala"> or whatever but that requires browsers to either depend on an external JVM or include on in the browser. Or you need to put the QML runtime in the browser or as a dependency. Or you have to put a Python interpreter in the browser or as a dependency.