The difference between path of exile seasons and Diablo 3 is that PoE adds mechanics, gameplay to it. One hardcore (season) version focused around bunching up enemies to kill which they spawned portals which could be chained together to summon bosses.
Diablo 3 seasons it just leaderboards.
Blizzard does literally jack shit with Diablo 3 besides recycling content. Rifts, Grifts, bounties, even legendaries (most new legendaries this season are just revamps of legends that had nothing) all have been recycled from the base game. Which isn't a bad thing but when it's the only thing they do....