Silvally has balanced stats. While this isn't a bad thing, it means it can't excel at any one thing, and thus has a harder time finding a niche. It can be good at quite a few roles, but that's all it is. Good. Not great.
Type: Null can use Eviolite. This boosts its defenses, and allows it to have an easier time Toxic-ing the opponent, lowering Defense with Crush Claw, or what have you. Type: Null is slower than Silvally, but it still serves a viable role (not great, mind you, it's still in PU).
So overall, Silvally doesn't excel at any one thing, and cannot use the Eviolite like Type: Null can.
Hope I helped!