BH, that's a good question.
The P&W is a lighter engine, and does normally give a little bit longer range. However I understand that the RR is a quiet engine, and has historically demonstrated greater reliability in service. I think the reliability of the RB211-535 series has been the overriding factor in chosing the RR engines over the P&W. I also think that on this aircraft (757), RR seems to always be first with delivery of the next higher thrust class engine when it was requested by Boeing, which doesn't hurt sales. The RR (RB211-535C) was also the first engine to be certified on the 757 when it was first introduced by Boeing. The PW2000 was a late comer.
In other words, RR seems to have always been in the lead with engines for the 757.