My own interaction with two developers there several years ago was that the web was clearly the redheaded stepchild of Valve. I think they said something like maybe 7 people worked on those features back then, one of those two only sort of worked on it. Additionally, they only really had something like 3-4 people focusing on it most of the time.

For years they had prototype.js included, yet the library was essentially dead at that point for at least a year. Now they have jQuery and are running 1.8.X version, which means they are using a 3 year old version. I think this kind of speaks for itself.