Because 2 stamp guns are cooler. I would think it is because most are dedicated to the gun from back in the day. My AWC MK9 slides over the barrel, and replaces the barrel nut to tighten up to the frame. In it's current configuration, I cannot attach it to any of my other weapons, where my Octane 45HD I can switch between many different guns.
I bought mine as a 2 stamper. Everything was submitted together at one time, got the tax stamp back for the suppressor, but not the gun. It was unsatisfying to get the call saying I could have the suppressor, but I could not have the gun as of yet.... Took 2 extra weeks to get the gun stamp. I was very nervous thinking there was going to come up with something wrong with the gun, and it wasn't going to be transferable. Then what was I going to do with the suppressor??? (AWC MK9) I bought mine as a package, and the deal was good enough that the additional $200 tax stamp did not affect my decision at all.