My point is that with every generation comes a new set of hardware, different from its predecessors and yet BC was still a thing up until this gen. I know not all games were BC on the 360 and the PS3 but it still happened. I'm sure if Sony worked hard enough it's possible for them in some shape or form but if they don't think it's feasible, then that's a separate decision.
The initial PS3's were backwards compatible because they had PS2 emotion engine CPU's in them. PS4's do not have the PS3's Cell CPU's in them, and the overall architecture is so different it's not an easy thing to emulate.