According to a post from a while back, it's some bullshit reason about barrel length.

I think of you look for the patch notes at around the time just before the release of TTK, you might be able to read up on why hand cannons got that bloom effect added to them. Before that time, hand cannons basically were the go to primary weapon to use in the crucible. Especially the holy trinity of exotic hand cannons. Being thorn, hawkmoon, and the last word. Legendary hand cannons during this time in destiny were also very strong, but paled in comparison to the exotics. Sadly the numerous range nerfs to hand cannons at the time just wasn't enough of a deterrent. So my guess is BUNGIE had to take a more dramatic measure to try to promote more variety when it came to primary weapons use in the crucible.

Because they couldn't just do the logical thing and alter range/damage dropoff/or rate of fire. Implementing the perception of RNG makes much more sense.

Because without it, nobody would use anything but hand cannons..

they're designed to be closer quarters, so they added bloom to try and prevent long range engagements since we have scouts or pulses for that

Because without it scout rifles would have no purpose