Two shot is just so much more powerful than every other affix.
One of them does 25% more damage, some do a tiny bit extra bleeding or elemental damage, one has 30% armor penetration, one does double damage on full HP enemies, etc... All of that pales in comparison to basically doing double damage at all times against all enemies with two shot...
A simple fix to make other affixes be more desirable is to nerf two shot, but it would be cooler if they brought everything else up rather than brought two shot down...
*That or just release the creation kit because we all know it is ready since the game is on the same engine and the tool is exactly the same as it has been since Fallout 3.