The reason people stop playing modes like Odyssey and how it might be fixed.

It's limited. You have a clear and defined ending to the mode. Once you beat all of the levels and get the rewards players will of course leave the mode. What we need is these modes to be expanded upon. How I would do it is this, first have a surviver mode on all of these events, where it's endless. Rewards, like an emote, given for reaching a certain point or wave. A high score table to show off you reward, just like a rank. The altered powers you gave champions to level and change was a golden idea, so include it again. Give waves where champions fall off, by that I mean don't make it so each level is 5 people always. Have people be separated and be forced to survive alone long enough to reconnect with their team. Those who can't maybe have a chance of coming back repeating the level or buying back system where lives can be restored by gold earned in game or they can just stay gone. You need to think about really expanding the modes. Maybe make it a tournament survival game. Teams survive level after level then have to 1v1 a different team, one death each, to continue. There are endless things you can do to draw attention to these games, but the worst thing you can do is give us an easy, medium, hard system of play and expect any interest to be drawn toward it.