More importantly, Kryptonite is also a SIMPLE tool to work with against Superman.Naw, giving him the option of ANY gadget in the world means he doesn't even need kryptonite, he can just take things like the Phantom Zone projector, red sun lasers, Green Lantern rings, teleport tubes, etc. Kyrptonite is just the easiest Super-stopper Bats has on-hand.
If it was red sun lasers, Green Lantern rings or other nonsense, there'd be a small chance Supes could use them against Batman. Kryptonite? He can't even touch the stuff, so that makes things much more simple.