At a guess, Liability. For Yahoo to state that they'd be keeping an archive of geocities up, there would be legal issues about what kind of guarantees they're

At a guess, Liability. For Yahoo to state that they'd be keeping an archive of geocities up, there would be legal issues about what kind of guarantees they're giving, legal state of the sign-up 'contract' that people entered into when they started their geocities page, legal requirements for proving someone is who they say they are when they suddenly say 'hey, delete my page I created in 1997, btw, I moved 8 times and changed my name 3 times', how that all fits in with EU data protection laws, etc etc. Geeks doing it makes it a 'fun project', with no expectation of those legal issues (not that some of them don't exist still). If it vanishes overnight, it's a loss of something that wasn't guaranteed, For an individual to do something is a different endevour from a large corporation doing something, and in this case, the financial burden of keeping a read-only archive of geocities might have been too much for yahoo. Or perhaps they never even contemplated it :)