http://www.theawl.com/2014/06/hbos-historical-revisionism

No “Dream On,” no Mr. Show,” not even “Larry Sanders.”
Read more at http://www.hitfix.com/tv-tattle#t93SzT8QCvh4Hord.99

My guess is copyright issues, related to the multiple clips from older TV shows that were inserted into each episode. Also, HBO didn't produce a lot of its earlier shows, which is why they don't have some of their earlier stuff available. In Dream On's case, it was produced by what is now Universal Television. So a lot of contracts would have to be renegotiated, which frequently doesn't happen.