I started riding PWC in the mid 80's when a couch was a rarity and I think "seadoo" was still only making snowmobiles.
The two 3 person machines I have are mostly for guests and to waterski behind. To me, going out for a day on a sitdown is boring unless I'm just going into a new area and want to site see. I MUCH prefer the challenge and action of a standup. The main reason standups fell from popularity was because they have to be light and the 2 stroke is pretty much a thing of the past in it's simplest form. The other reason is that it actually takes skill to ride one. There is a learning curve and most people are not interested in investing the time to learn, they would rather just sit there, squeeze the throttle and ride fast. Plus, they can't ride a standup and hold their beer at the same time.