kuduae is correct. The regulations were written in states where previously hunters had to use shotguns.

The history is rather interesting.

As late as the '70's, buckshot in these states was very popular and some had only recently opened to slugs. Then the fully rifled slug barrel became popular especially for sabot slugs.

That tested the laws and eventually more or less it was made legal as the ballistics were still similar enough to shotgun smoothbores.

Reality was the sabot slug guns were producing ballistics more or less equal to the .45-70 and the legal game was on. Pressure to allow some sort of rifle was permitted. But what? Enter straight-walled "rifle" cartridges. The with that came a growing number attempting to gain traction in that ever-increasing market. Some of the states w/ these restrictions have some of the largest deer populations and now pig as well. The increasing use of the AR15 added increasing incentive to design rounds suitable for that action.

Interestingly, there are some .223 rounds fully capable of killing deer and some guys use them in various stated where the bottleneck is allowed. I know of some that have shot thousands of deer in control actions using heavy bullets in .223 in AR15's. But w/ the bottleneck outlawed, that option is gone.