A lot of people, when they find out I dive, immediately say, "Oh, I could never do that, I'm too claustrophobic." I'm not sure what there is about floating around in a huge body of water that's claustrophobic, but apparently the image is to a lot of people.
I really don't understand the many, many people I've met who got certified and never dove. Some never did a single post-certification dive!
I do understand why people would choose not to dive where I dive, or where a lot of inland folks dive. The heavy exposure protection, the massive weight, the reduced visibility, and the knowledge that most dives will end with you cold and uncomfortable are good reasons not to pursue our local diving, despite the richness of the underwater environment.
I wonder if another barrier is that most other things are things you can try without committing to a class. You can rent skis and go skiing with an experienced companion and give it a try, and take your lessons later if you want. You can't do that with small airplanes, but not that many people fly. Unless you're somewhere where "Discover Scuba" is available, you can't do it with scuba diving, either, and I think people are sometimes reluctant to part with the costs of a class when they aren't sure they're even going to like diving. I know I felt that way.