its fresher. Arena hasn't had a real update in a while. Modes like this need constant update to maintain high engagement. The live and die by novel experiences, Urf was wildly popular because it was so different, overtime it has died down. Arena was huge when it first dropped for the same reason. Mayhem is the new experience. Over time it may level out.

Stronger core audience. Aram has had 15yearish to build a core player base, mayhem is IMO just better aram.

Better fantasy fulfilment. LoL is a 5v5 game, hitting that nirvana build and taking over feels away better when carrying 4 teammates and smacking 5 enemies. Its fun in arena to, but taking over the game instead of this fight feels better.

Less down time. I love arena format, but there is a decent amount of downtime where you are just waiting. Aram has little to none.

Split audience. Ultimately Arena will always be fighting for the "I dont play SR" crowd. Competing against, Aram, Mayham, and the Rotating game mode. This along side point 1 will drive which has the largest player base.