If you look at what's available in stores, the selection is pretty limited. Lots of Denise Austin, lots of "Dancing off the Pounds" guy (his name escapes me for some reason, some Kathy Smith, some Leslie Sansone and other Goodtimes titles. Most are beginner level, with some intermediate.
At some point, retailers decided that the majority of their customers who are looking for exercise videos are looking for that level of exercise. Also, it's probably easier for them to buy from just a few wholesalers (like Goodtimes, which has Leslie Sansone and the Firm, among others).