Richard, No good reason. Just another feature that a politician can make a firearm non compliant for.

It is because in your socialist state they think that you will put a silencer on the gun and if you can't buy one with the threaded barrel you can't put one on your gun. It is called gun control and people control.

It's because politicians in those states watch to many movies and believe that silencers/sound suppressor make no noise ( which isn't true) and are only used by gangsters to kill people ( which is also not true).
Same garbage as states that limit magazine capacity crooked politicians want to control their subjects and use false information to make people believe the stuff they are banning is for their own protection.

Unfortunately, the 2nd amendment is the only amendment to the constitution that is regulated at the state level. Therefore, each state is in full control over their gun laws (as long as it isn't more lenient than the ATF federal laws). Therefore, some prominently democratic states have more strict gun laws than others *cough California cough*
The logical answer, however, is because they want to limit the ability to add a muzzle device. Even though legally acquiring one isn't as easy as it sounds.

Richard, No good reason. Just another feature that a politician can make a firearm non compliant for.

Richard, as a deterrent to illegally adding a homemade suppressor/silencer to the firearm. Kinda silly as threaded barrels as a component are easy to come by. Cheers and stay safe!

the politicians in those states passed laws that said that you could not be trusted with that gun because it has a threaded barrel and you may break the law and put a silencer or other muzzle device on the gun

The ability to add a silencer.