The classification of something as handgun vs. long arm ammo is extremely soft and discretionary. As soon as one company makes a production handgun that fires a specific cartridge it's enough to get it deemed as handgun ammo.
Another thing to consider is penetration of the projectile for safety considerations. There are already hunting zones where rifle is not allowed for safety considerations, if you were forced to use steel slugs in a shotgun it would also reduce safe hunting opportunities.
It doesn't help that there is limited study about casual contact with lead. Most sportsman understand the risks of commercial levels of lead exposure but the occasional contact with the human body or environment is not easily quantifiable or understood.
In NJ for example it is a felony to posses to hollow points (NJSA 2C:39-3(f)) so you will likely face an additional felony if you are ever charged on another gun crime.
The projectile of a piece of ammunition is also used by the feds as an excuse to ban importation if they deem it not suitable for sporting purposes.