I think part of the problem with Spaceballs is that it's not just a Mel Brooks comedy, it's a satire of a specific movie. Blazing Saddles still cracks me up every time because it's a parody of Westerns in general, and racism, and Hollywood, and so on. But by targeting Star Wars, the humor is much more limited in scope, depends on knowledge of the original source (which admittedly isn't really a huge problem) and whether the viewer hasn't heard the same jokes 1000 times already.
I too remember liking Spaceballs a lot -- it was one of the first VHS tapes I got, way back when -- but when it comes round on TV, I can watch about five minutes of it before I have to change the channel.