Backward compatibility is not as simple as that.
With the X-Box, 360, and the One, that's not clear to me, I didn't follow that as it was progressing. But the PS1 though 4? I know a little bit of that. The PS3 was a new technology, B.C. was very expensive to maintain so with the cell processor it was a matter of incompatible technologies. The same reason applies to the PS3 to 4, the 4 has the power but not the same technology, like the difference between Lenox, Mac, and Microsoft. However, the PS4 is compatible to the PS2 and 1, the technologies are compatible and the rumors are that they are working to make it happen and to upscale the PS2 to render the games to 1080 and 720, like the PCSX2.
There might also be the reason that they want PSNow to have more successful run by limiting your ability to play PS3/2/1 games on the PS4. If you already own the game how will they be able to charge you for it? I hope I'm wrong but there is still that possibility.