Especially when two products are close to functionally interchangeable, like Ruby and Python, design can be decisive. Engineers chronically underestimate the importance of design and marketing.
Now guess which community has better graphical design skills? Or from the other end - all other things being equal (and of course they aren't in reality, there are many small differences), I'd rather use a language designed by someone who specialises in designing programming languages rather than one creating websites.
Are you referring to PHP or something? Certainly Matz is every bit the language designer Guido is. Like it or not, Python and Ruby are competing in a lot of very similar niches. Fluffing the design of your webpages is like stopping at the 24th mile of a marathon.
Doesn't sound like someone who specializes in creating websites to me.
I do however think that high popularity in one specific circle (rails developers) results in pretty pages about the language itself. The split was actually very clear not that long ago even in ruby itself. A couple of months ago docs for ruby core were on a really ugly and not very usable page with 3 small scrolling boxes on the top and documentation view below. It's gone now fortunately. (old version of rdoc generator theme - still visible on http://stdlib.rubyonrails.org/libdoc/rdoc/rdoc/index.html)
Disclaimer: you should't base your credit card use only on design!
These Python sites are targeted at Python programmers. What do Python programmers want? They want clean, neat and obvious. Above all they cannot abide unnecessary clutter.
The Ruby screenshots listed have the following 'visual impact' features. They may make them more attractive, but don't help with usability and potentially get take up space that could be used for information:
Elements introduced purely for decoration.
Oversized logos
Pictures of people
Designer fonts
This is a pretty rough analysis, but perhaps it explains the difference in approach?
It's only 'likely that they sweated the other details' if you assume infinite manpower and resources. In reality, all of the very finite resources that went into picking the right shade of beige for the background could have instead gone into improving the actual tools.
I don't think it's possible for an engineer to underestimate the importance of design and marketing for an engineering tool.
Have you seen nginx.org?