you can't use disk wheels or aero bars in a group for safety reasons. the wind could catch a disk wheel and blow you into the other riders, and aero bars don't usually have brakes reachable from the aero position.
the tt bike would probably be too uncomfortable to ride for hours at a stretch, whereas a tt is never usually much more than a couple of hours.
aero helmets are for aerodynamics only, and are not crash helmets, which are required to be worn during mass-start races.
hope that clears things up…