Money. Bottom line. It ain't a poor man's sport.
Kind of disagree there bud... IMO cost is the lesser reason.
Compared to what? Motorcycles, Photography? Only a few ACTIVE Sports / Hobbies are cheaper than diving on the mid-long term. I used CAPS on ACTIVE because of course, collecting post stamps is cheaper but also doesn't require any equipment. Team sports like Basketball and Football are also cheaper but you still need a court or field.
One can pay around 1K to get his basic diving gear and use it for 20 years or more if he dives an average of 30-40 times a year (obviously depending on use and conditions some gear might need replacement earlier).
Certification isn't uber cheap but I wouldn't consider 400-500€ expensive to get certified.
I mentioned Motorcycles and Photography because I do both. A motorcycle license in 2022 costs 1.500€, add another 1.000€ for
very average reputable gear and you've already spent almost 2 times more than Diving.
An modern entry level camera with a basic kit lens is around 1.000€ (I'm talking mirrorless as DSLRs are soon becoming obsolete). The moment you decide to buy some additional lenses,, things become quite crazy really fast (I've spent more than 20K but that's another story). Add to the fact that camera bodies become 'old' after 2-3 years as technology advances much quicker compared to diving equipment and you get the idea...
I believe fear of the unknown (huge man eating Octopuses and Sharks), lack of buddies to share the passion, time (you need the whole day to make a proper dive + the post dive debriefing & festivities usually on a tavern) and / or country of residence, meaning ease of access to water.
In Germany, finding warm, salty water is basically impossible so basically either you dive in cold lakes or travel many Km in older to get to the sea. Result: I don't dive as often. When I used to live in Greece I was diving every weekend.