Even in a state where suppressors are legal, they're NFA items and most people don't want to jump through the hoops to get one. Their only opportunity to use one would be borrowing a suppressed gun from a friend at a range/in the woods, and the friend wouldn't let them install the suppressor themself so the average person wouldn't know that they're not plug and play with any kind of gun. I would guess the shooter bought one of the "solvent traps" having never used a suppressor before, maybe never practiced with it because he didn't want anyone to know he had it.
I would expect the cops to have more of a lead on him in this case. If he had any brains he bought all his gear through proxies, including his clothes and backpack.