The selection tools in PS are in a class by themselves, with huge improvements over the last few versions. Add to that things like mask refinement and non destructive mask blurring and many other refinements and there seems to be very little real competition for PS right now, at least for those who rely on masking and layering.
And the unfortunate thing about those refinements is that many of them are covered by software patents. So it's not just a matter of cloning the functionality. In many cases a new piece of software would have to use significantly different paradigms and techniques, hugely increasing the effort required to achieve similar functionality.
I invite the users of PS competitors to correct me on this, but it seems like Gimp and some other PS-like programs are using something closer PS5 as a model, rather than the later CS versions. It occurs to me this may be partly because of patent issues, dating from the unfortunate day in the 90's when software patents were first granted.