Because when WWE wants to promote a wrestler and treat them like a star, there's no other company that comes remotely close to elevating that wrestler's value. And when that performer is talented at something (charisma, mic work or in ring), it's a flawless formula.
Cody came in carrying himself like a star, knowing how to cut a good promo and showing a fukk ton of charisma. There's no world were WWE misses on elevating a guy like that if they decide to actually put the effort in supporting him.
AEW has literally none of that. They don't have promoting experience nor the power to elevate anybody. They just let wrestlers do whatever the fukk they want and sink o swim. In Cody's case he stayed too long in his own corner booking his own stuff and it backfired. In WWE's case they've managed Cody's strengths and elevated them, which is not something they usually do, but when they do....it's a 100% hit ratio.