It also has a lot to do with there being very poor ways of managing dependencies in Javascript. For instance, in python there is pip/pypi and in ruby there are gems. Javascript doesn't yet have that. I don't check in all of my dependent libs into my repo. I do check in the Javascript libs though.

There is npm (http://npmjs.org/) but it requires node and for most people, they don't want to bother. They just manually change the name of the file or directory and check in a new version.