I shoot 9mm and .45 and I own a restricted KRISS in .45 and I love it. Besides the "knock down power" I think for alot of people ammo price difference between .45 and 9mm would be a big factor, and more people would be interested in the firearm in the cheaper to shoot 9mm caliber
I wouldn't give up my .45 for the 9mm, and they did an amazing job making the .45 easy to shoot accurately and rapid fire out of the Vector, that being said I just ordered 1000 rounds or 9mm and it cost me $416.02 shipped to my door, last week I ordered the same brand 1000 rounds of .45 ACP and in comparison it cost me $607.34 to my door. So yeah for some the almost double the ammo price might make a 9mm KRISS Vector seem like a good idea
Especially when your doing this all day
https://www.youtube.com/watch?v=mcEwPg-q_00