He just does. Very different moveset. King and Armor King really only have the looks and being wrestlers in common. They do share a couple of moves, especially in T7, but overall they are a lot different.
Edit: Or I should say that King in T7 has quite a few moves Armor King used to have. We'll see how AK will turn out in T7.
Armor King is completely his own character despite looking similarly. He plays much more like Jin than anyone else in Tekken. In my opinion, he's more like Jin than he is King. He's been called a "shitty Mishima" and that description does fit. He's what you'd get if you gave a Mishima pro wrestling moves and a grapple mixup. He's not as strong in neutral and poking as Jin but he gets a ton of benefit from the Mishima wave dash mixup because of his throws. So not only can you wavedash uppercut a fool with his Dark Upper, but you can dash up and Giant Swing, combo throw, dark lariat or hit a power mid.
And his WS4 used to be probably one of the best in the game, which is great because you can just spam that shit. People always want to duck against Armor King because his scary moves are duckable. But if they duck, you can just cd4, ws4 or ff2 them. Forgot to mention he has a Demon Paw which King doesn't have. That gives him another scary option, and it's mid which is even better.
He also has stomps (d444) which are guaranteed after certain moves and will piss off your opponent. I usually do babby combos and end with stomps and an attempt to pick up using d3+4.