The reason for not setting the park brake following an RTO (and we are really talking quite high KE not a low speed abort) is to prevent the hydraulic thermal fuses from blowing which prevents overpressure in the park brake hydraulic lines.
As a previous poster has said when the pb is on the return line is isolated and 3000psi is trapped at the wheels.
A secondary reason is the paranoia that the brakes will seize due to high temps and not release on selection.
Also not setting the brakes allows air to get between the rotors and stators and cool everything much quicker. My experience from RTO Flight Testing is that brake temps can continue to rise by up to 50 degs C with the brakes still applied vs immediate cooling with them released - less chance of fusible plugs blowing.