I asked this on Smogon and they said
Population Bomb checks its 90% accuracy for each hit, so its chance to hit all 10 is 0.9^10 which is about 35%. Wide Lens brings the accuracy up to 99% for each hit which increases its chance to land all 10 hits up to about 90%. Since it relies on the move so much, you really want it to be consistent, so it doesn't really have another choice.
It only guarantees 4 hits, the rest are still inconsistent. It has seen some experimentation in the past with Bullet Seed for Ground and Rock types, but it never truly caught on.
As for FrostZilla, it's because of scale shot and icicle spear. Anyone who has played gen eight especially when the meta became stable after the Magearna ban in CT would have used scale shot Garchomp and would also know how fking infuriating that move is when it hits only twice. The same goes for icicle spear when Kyurem Black was still in the tier. FrostZilla, who can learn both these moves as its stabs, prefers loaded dice so that it has a consistent one hundred base damage move, one twenty five if you got lucky, and both stabs have minimal drawbacks. Scale shot even boosts speed which is massive on FrostZilla whether it goes dd or sd, though sd is the better partner for it. Of course, the major drawback is FrostZilla is an ice type meaning rocks but that's often something players are willing to pay just to get consistent high base damage moves
