For me, it was a matter of challenge. I flew 1 contest in Advanced, and just didn't find the maneuvers that challenging. So the next contest, I flew Masters. That gave me what I was looking for.

Same for IMAC. I flew my first contest in Advanced, won it by a LARGE margin, moved to Unlimited and never looked back.

The advice people have been giving you is solid. Go with what's FUN for you. This isn't an ego thing. There NO shame, or whatever from flying ANY class. If you enjoy flying Sportsman, by all means fly Sportsman. Don't feel like you have to move up for any reason other than you feel you're ready or want to.