With this game, it is not appealing to start over completely fresh every season. For one thing, having a large collection of competitive pokemon actually makes the game more fun because it adds more options and complexity to choosing a competitive team. The other (relevant) difference between Diablo and PokeMMO is that you actually use your pokemon (equipment being the equivalent for Diablo) to create better ones. So, in Diablo at the conclusion of a season, tons of really nice and powerful items get converted to non-ladder items, and it pushes in a nice influx. At the conclusion of a season in PokeMMO, it would create an influx of mediocre pokemon that weren't bred using the dittos and godly pokemon players have accumulated.
Instead of the treasure galleon sailing into port with exotic spices and silks, we would get the trash barge crashing into town with all of its filth.