Could be to discourage people from storing media there. OS images and videos are probably the biggest things people would store. They're also the most pirated content. My guess is either: a) they don't care to host 100TB of pirated content, or b) they're afraid of the legal ramifications of it. Imposing a file size limit keeps a huge bulk of usage away and a lot of the piracy concerns while not really inconveniencing the vast majority of normal users.