Because seasonal play is temporary. If you end up liking a season or mechanics from it, well too bad. It’s only there for a few months, and if you are lucky, you might see some aspect of it show up in the future.
I would rather play with my one character that I chose at the beginning, and upgrade them continually. I dislike continually remaking characters, and usually only end up with 1 of each class in a game, and a couple of my favorite ones on different game modes.
Overall, seasonal play is garbage content for the most part that adds nothing to the game other than a temporary bandaid to cover other issues, a distraction.