True, really the ones who can answer that are the ones who want to dive but haven't taken the plunge, and I don't think we'll find many of them on this site. Cost may be a bigger factor in them. The significantly higher than median income does at least appear to support that those people have a bit more disposable income to support this hobby. Not always the case, I know. People of all income levels can have a lifestyle that is above their means.
It does have a higher cost of entry than many other outdoor activities, though probably not the highest. Hiking, fishing, kayaking are all activities that you can get started with with a much lower initial investment. I do those as well, but if I had to give up all but one, I'd keep diving. When I'm out on the water for other activities, I find I really want to get underwater.
Agreed on the renting when starting out, but that presents another barrier. Now, I need to plan farther ahead. I need to make sure I can reserve a set of gear and tanks, get to the shop the evening before (for morning dives), and also plan to return the gear the next day. When owning, you have quite a bit more leeway on that. My cylinders are all currently full, and ready to go.