My theory is that they sit on a mountain of technical debt that nobody dares touching. I think they started in Perl, then URLs started pointing to DLLs (!), so heck knows what they've done...
They were one of the very first websites that had to deal with humongous scaling issues, so I'm not saying they are stupid - just that, after a certain point in time, they probably ossified at a level that makes meaningful progress too difficult.