Well, I think for two reasons:
1. They are anatomically correct, which means nothing gets in the way of the shredder's playing. One must here remember the practice regimen for which shredders are notorious - if you play for 12-14 hours a day you want all the help you can get in terms of avoiding strain on muscles etc.
2. Bolt-on gives the most immediate response to one's playing, which is important when playing fast.