And my intuition is that this is because the integration of nice CSS and HTML templates with code libraries makes Rails a real breakthrough for individual programmers who can create nice looking sites without hiring web designers.
I may very well be ignorant, but I didn't realize that Rails came with a web-design-in-a-box like that. Last I looked, the skeleton code is pretty decent, but far from "designed". In the end though, if you want to create a professional product, you will have to hire a web designer, and that is something that the Rubyists do seem to get more than any other language community.
I think that while this is a good point, some of the new CSS "frameworks" like Blueprint, YUI Grid and more might make it a moot point. Also I have seen a trend in JS frameworks (in particular with JQuery which we use at work) to make it "look nice by default".