Web is the most accessible medium. If you can view it in your browser, so can anyone else. There is no complex configuration or build process (though it seems the JS community is dead set on changing that). What You See Is What You Get. It's an immediately useful and intelligible medium and you mostly stay away from the really scary things. Teaching simple web dev first makes sense.

Rails is probably chosen mostly out of the false tradition that Ruby is a good beginner's language or the also-false tradition Rails is a good framework.