I hate single supplements as much as anyone else, but I understand why they are applied. It has to do with profit.
The cruise line typically makes little profit from two people in an ordinary room (let's disregard top-end suites for the moment). It may even lose money. Overwhelmingly the profits come from beverages, casinos, beauty parlours, shops, excursions, Internet service, and other things on which the captive passengers drop a lot of money. Cruise lines have abundant data on expenditures made by passengers in different categories; they know, for example, that two people travelling together will tend to spend more money on those ancillary things than two people travelling separately. For that reason (which again is separate from the fee for the cruise itself), double occupancy is much more attractive to the company than single. Something about the dynamics of travelling as a couple causes couples (broadly construed) to spend more money.
From our perspective, although we occupy a room that could otherwise have accommodated two paying passengers, we don't consume the food of two people, nor do we cause so much wear and tear to the facilities or make such great demands on laundry and other services; thus it seems unfair to charge us double. From the company's perspective, even if we pay double, we still bring in less money than two passengers in the same room. We are therefore undesirable even at a 100% single supplement.
One problem with that position is that it leads to discrimination. I've seen single supplements in excess of 100%, meaning that a single person pays more than two. If that is to be permitted, why not perform the same analysis by race or gender and charge higher fees to groups that tend to be less profitable? That would be unacceptable, but cruise lines get away with handling single occupancy in precisely that manner.
Offsetting our undesirability as individuals is our desirability as a group. There are many more single travellers than there used to be. Now that 43% of Canadian marriages end in divorce, this should be unsurprising. That's why cruise lines have shown more interest lately in us long-disdained single travellers: we're just too large a group to neglect. So single supplements are being reduced and other overtures are being made to attract us.