There's another thing that goes against gliders and transponders, multiple radios and the like - POWER. Specifically, everything must run off batteries, and when flights are frequently in the reigon of 5-10hrs, that can be a real issue.
I also rather doubt the practicality of working under ATSOCAS for gliders - they don't fly in straight lines, maintain altitude, speed, or anything like that for very long; if you were to tell someone every time you changed height/speed/direction, you'd never stop transmitting.. They also tend to want to look out the window, and enjoy flying, not play at airline pilots(!) Yes, I have a foot in both power and gliding camps..
@ShyTorque - In Aus it's pretty common to hear the 'heavies' ask for (and receive) "high speed climb", or "cancel speed restriction below 10,000" - As I understand it, the ATSU has the authority to allow as they see fit. Passing in and out of controlled airspace - er - goodness knows!