Via MR, I found this page on condom pricing. The author observes that you can obtain condoms at substantially lower per-unit prices by buying in large quantities (especially if you order in bulk online), and then suggests several possible reasons why people don’t take advantage of these lower prices. I think all the reasons listed (especially 2 and 4) play some role, but I would add three more:
1. The psychological effect on new partners. What will a new partner think when she discovers the box of 500 condoms in your closet? Maybe she’ll praise your frugality, but more likely she’ll wonder about your sexual history.
2. The relationship factor. Single people presumably have less sex than people in long-term relationships. (Seriously. Look, for instance, at the numbers for single versus married people; popular myths aside, married people do it a lot more often.) So people in long-term relationships are presumably the best candidates for bulk-buying. But a lot of couples will, after an initial period of condom use, get vetted out for STDs and then abandon condoms in favor of chemical contraception. And even single people relying on condoms might anticipate the possibility of a future relationship that makes condom use unnecessary, which argues against stocking up too far in advance.
It’s also worth noting that the effect of volume on condom prices is large, but not unheard of. For example, I found that 3.1-ounce bars of Ivory soap cost $0.63 each when you buy a pack of three, but only $0.38 each when you buy a case of 96. That’s a 40% reduction, which is comparable to the approximately 50% reduction in the per-unit price of condoms for the same difference in volume. But I suspect not many people buy 96-bar cases of soap. Maybe they would if they were more farsighted, or maybe they’re wary of filling all their closets and cabinets with all the items one could justify buying in bulk. Point being, we don’t need a condom-specific reason if people have an aversion to bulk-buying in general.