#1. It was the safest choice for him

Omega did all he could do in NJPW. He won the top prize, always had solid matches and was a member and leader of one of the most influential groups in wrestling history in the Bullet Club. From the Bullet Club, the Elite was formed. It was the whole basis for the Guerillas of Destiny turning on the Elite.
When all was done with negotiating, he ultimately chose to stick with his friends in cultivating their own promotion. They all would have backstage roles in the company and obviously a huge say in what goes down in the promotion.
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But during one interview with the Sporting News, the Young Bucks said signing with AEW was the easiest or safest choice. They'd be able to have total creative freedom and be able to still have fun with all of their friends. That's something that is certainly important with wrestlers, otherwise they might ask for their releases from their respective companies.
It was certainly the easiest and safest choice, but sometimes those aren't the best choices. For them and their families, the Young Bucks made the right choice. They can do basically whatever they want in AEW and dictate their schedule. But Omega signing there keeps him in his comfort zone with the rest of the Elite.
If he truly wanted to challenge himself in a completely new way, it would have been as a member of the WWE roster. Some fans might have feared that he wouldn't be booked like a top star, but from what he said about negotiating with WWE, it sounds like he and the Bucks would have been treated like big deals. Who else gets a potential invasion angle at WrestleMania?
Sometimes jumping out of your comfort zone forces you to up your game or improve your character, so to speak. By staying where you are comfortable, you miss out on the rest of the world and what it has to offer. By passing on WWE, Omega chose to stay with whom he was comfortable with although by changing the location. It might have been the best choice at the moment, but who knows how he'll feel 20 years in the future if he never gave it a shot in WWE.
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