As far as I've come to understand..older TV shows were shot on film which can be made into HD..but anything between 1985 and say 2000 or so was on tape and would be costly to turn into HD, if not impossible. Film has more resolution to it than HD has, which is why you can get HD converts of movies not shot in HD to begin with as well. And they are cropped at the top and bottom but have more picture on the sides since the film was cropped on the sides and left at the top/bottom for square TVs. And the HD quality sometimes looks bad because the original was bad but not really noticable due to standard definition.
Which is why you can watch Hogans Heroes and Charlies Angels on HDNet, and its possible for a lot of the older stuff from the 60s and 70s if there was demand for it, demand being the key word.