Because Jeff IS (was) Slayer. So is Lombardo for that matter, and I don't see him in the picture either currently. I am not a Slipknot fan so I can't comment, but comparing one to the other is apples and oranges. Each entity has their own separate situations and lifebloods.
Lots of people weren't upset when KK left Priest, they tried to justify it by saying he wasn't an integral member, I disagreed. I'm sure that still can be debated.
But Hanneman wrote all the good music for this band. Would Maiden function right without Murray? Or Metallica without Kirk? They could do it, but a ton of eyebrows would be up in the air like the Rock.