A lot of these specialty channels had to get either FCC or CRTC approvals to broadcast and, to get this approval, one of the things they had to prove was that they were providing programming not available on convential TV stations. hence the creation of a specialty channel for sports, news, history, science, nature etc....
But, of course, what happened was that no one watched them (with the exception of news and sports) so the programmers did what they had to do, program down until they found a sufficient number of mouth breathers to bring in some advertising revenue (AE has probably strayed the farthest from its mandate).
These subscriber based services are mostly owned by large media conglomerates and they found this was a great place to dump their inventory that was sitting on the shelf (red dawn fer example) and still make money off it.
what once was an opportunity to get some diversity onto the airwaves has simply turned into same shit different package so that it is now almost impossible to differentiate between TNT and AE.