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Muk and Kingler are unique Pokemon, (well not Kingler so much anymore), that can be used along with his other Pokemon. Ash has no poision sludge Pokemon like Muk, there's no reason he *shouldn't* use it again.
It's not being used for the same reason that Kingler and most of his jotho captures aren't used anymore. They were handled badly and there isn't much else they could do with them now.
Muk and Kingler are unique Pokemon, (well not Kingler so much anymore), that can be used along with his other Pokemon. Ash has no poision sludge Pokemon like Muk, there's no reason he *shouldn't* use it again.