Economics. Altitude costs money; the more you get, the more it costs. For most jumpers the extra altitude isn't worth the extra cost on a regular basis.
Big way RW groups routinely jump from 16-18k. They're on oxygen from about 10k until just prior to exit.
Some dz's offer higher altitude jumps - Skydance in Davis, CA does 30k jumps (using bailout bottles) once or twice a year. They're supposed to be going to 34k this weekend; a guy is going to attempt to set a wingsuit record.
I don't care about the money part of it. I'm just saying... if after 15k its supposedly harder to breathe, with skydiving your gonna be at that alt for a much shorter period of time as compared to paragliding up to 15 or 20k, then slowly gliding down.
btw...i think it would be cool to put on a skydiving rig, attach the paraglider find a strong thermal get up to about 15k and cut yourself loose
