I apologise in advance if this is going to sound provocative on a forum and thread with "SCUBA" in the names, but as a lifelong snorkeller who qualified in SCUBA in the late 1960s but hasn't engaged in SCUBA diving since, I think one issue needing addressing is the assumption that self-contained underwater breathing apparatus is central to all underwater swimming activity. Maybe wrongly, I get the impression from many posts that the swimming and snorkelling skills I had to master fifty years ago are no longer considered necessary for anybody aspiring to SCUBA.
I snorkel because I live just eight miles away from the sea and I can spontaneously jump into my car with my snorkel, fins and mask and be in the sea within twenty minutes without having to worry about air fills. When I'm abroad, it's easy to throw in my basic gear and go for a dip in the nearest river, lake or ocean. I can go on vacation wherever non-diving attractions (history, culture, people, language, food, shopping, architecture, nature) are paramount rather than locations catering for diving alone. I can choose to snorkel from public beaches, like the locals, rather than being segregated and herded around on tourist boats.
So I'd ask the question "Why don't more people take up open-water swimming and diving?" rather than limiting the topic to SCUBA, which is one water-based activity among many. Let's celebrate people getting into any outdoor aquatic pursuit, rather than just singling out SCUBA diving as the "Holy Grail" of aquatic activity. I enjoy snorkelling because it's a simple, inexpensive activity that I can still enjoy now that I'm in my late sixties and appreciate a quiet life. I respect those who do SCUBA, but no thanks, I have no plans to join you in the activity.