Because Locusts are good anti-everything finisher. Also very reliable and has lots of ammo compared to other launchers, especially if you vent to control number of missiles launched. It even costs less OP than other heavy launchers (if its OP cost did not change since 0.8 ). Once a big ship loses armor, it takes a ton of damage from a Locust burst. Frigates die outright to one burst from two Locusts.
Yep, nerf Locusts.
They're far too powerful for how flexible they are.
Though it's not a nerf I'd do in isolation, there are a number of assault-type missiles that need looking at too; Sabots, Harpoons, and all the torpedo-armed bombers.
Bear in mind my perspective is based upon battlesize of 500.
Missile & bomber balance is less of an issue in smaller engagement sizes.
As increasing PD range isn't possible (as it'd require increasing the range of all other direct-fire weapons), I'd suggest reducing velocities of these assault-type missiles by ~25-33%.
It'd increase the effectiveness of both PD & basic manoeuvrability, without impacting much else.