I think part of the problem with JavaScript, php, perl, etc is that is there, by default, everywhere. A great many beginners do some very ugly things with it because it's already on the system they have and there's tons of not so good examples on the Internet to cut and paste. I'd bet there are plenty of people creating web sites who aren't entirely clear on where HTML stops and JavaScript starts.

Ruby, for example not only has to be installed on most older systems, you'd have to know that you wanted ruby in the first place. This alone acts as a giant filter. Anyone who knows they want to use ruby is already going to produce better code.

Edit: Please s/ruby//g with anything slightly more obscure than perl/php like python, go, object-pascal, etc. I'm not trying to be a ruby elitist here. It was just an example.