Sony have set the SRP for their PS3 titles as £39.99. What third party publishers choose to set the SRP for their titles at is up to them - it's not actually set at £49.99, but most publishers seem to be opting for this price point (probably because it keeps the price in line with their Xbox 360 titles).
Blu-ray and HD-DVD are different beasts entirely. What you're effectively saying is no different from asking, say, "Why can't all DVD's have the same SRP?" - it's just not going to happen.
As others have said, the movie studios set the SRP for their titles on Blu-ray or HD-DVD and it is not a matter for the Blu-ray Disc Association or DVD Forum.