Why Stainless Steel Boxes Hold Less Odour
Plastic litter trays smell after a year even when they are cleaned diligently. It is not your cleaning.
Scratches hold residue
Cat claws and scoops score plastic. Those scratches trap urine and bacteria in places that wiping does not reach.
Steel does not scratch the same way
Stainless is hard enough to resist scoop damage and non-porous, so it wipes back to clean. It also survives boiling water, which plastic does not.
The trade-offs
Steel is heavier, costs more up front and is noisier when a cat scratches. Against that, it is usually the last box you buy.