First of all, before everyone jumps to conclusions, like many of you are, the parity is not Xbox One vs PS4, it's because it's on last gen, "We’re cross-platform, so we had to be very delicate with all four consoles."
However, even if people want to argue why isn't it 60 FPS, you have to also ask why Drive Club is only 30 FPS.
Destiny is a beautiful game with vast worlds and lots going on at certain points. A solid 60 FPS probably wasn't feasible at this time. And I say "at this time" because history shows us 100% that it takes developers time to tap the full potential of new console hardware.