Simple, he was exposed.
Mcg was always a counter puncher that guys couldn't get his reach and for some reason his left hand,(which means a lot about gameplanning and style analysis in mma) he got favored by dana the the UFC matchmaking while getting title shots by his obnoxious shit talk. Once he got a real threat he crumbled with khabib and he struggled against Nate, not to mention his horrid cardio.
Regardless of how mcg was exposed, it's amazing how much time it took for a fighter to expose his style which it took a freakshow in boxing to expose his striking, we all know his greatest weakness was wrestling but striking wise we haven't seen it until the khabib fight.
Mcg is just a overrated fighter who got favored by the ufc.