Is it that big a deal, really?
Funnily enough, I texted that to a mate barely an hour ago. I think it feels like bigger news that it is because of his history with the company, but if we assume that a 2 year extension was on the table (the same as the Bucks announced at the New Year), then what would another 2 years of Cody have looked like, really? Losing to then beating every exciting new signing? A TNT reign every few months? He can be rightly proud of his first year in the company - the match with Dustin, his title shot on the first PPV (the Jericho feud was literally the only thing involving promos in their first few months), and the MJF match (even if just for the 10 belt shots causing Brandi to drop the Nightmare Collective thing). He was also one of the few senior talents still available in those first pandemic months, so did a lot to hold things together there, too.
But something has spoiled since, and whatever he's been going for with his 'I won't turn' stuff, the fans aren't going for it. AEW's roster has grown and refreshed several times over in the last 2 years and any spots he had in main events or PPVs are being supplanted by names coming in at a higher level. Punk, Malakai Black, Danielson, so on.
At the same time, AEW's young, fresh faces like Jungle Boy, Darby Allin, Sammy etc have matured from hot prospects into solid mid-card talent, and a whole fresh batch of starlets like Garcia, Moriarty, Dante Martin have taken over at the start of their own journeys with the company. And to further add pressure to someone at Cody's level, Tony Khan keeps squeezing surprises through the forbidden door for one-off or brief high-profile shocks.
Rumour is that money was the deciding factor, and if so then fair enough. Cody places a higher value on his brand than AEW does (given the above), and he's got more options than he did 3 years ago: Take a possible WWE contract as the first 'defector', or lean into his non-wrestling celebrity, or even just go on a Matt Cardona-like tear through whatever promotions he wants.