Like all projects, I guess someone has to actually maintain them. Remember that these people are almost never in tech at all. They probably spent quite a bit of time figuring how to put up the lecture series the first time. After a few years, they might not even remember how they did it. If a service winds down, or the university IT department makes a change to the infrastructure, they're probably not going to have the same motivation to learn how to do it all over again just to keep some old lectures up online.