drop out rate for divers was something we talked about around the dive shop where I worked in Socal in 1980s; my hypothesis would be based on the lamentations I have heard over the years...
1) bucket list --people want to try it like parasailing or sky diving; then never do it again.
2) lack of a buddy to dive with. even in a club setting lots of people don't want to dive with people they don't know. I never had problems finding people who would hike or ski with me though at a club.
3) logistics of getting the gear (if you don't have your own), getting to the site, and getting the gear back.
4) attitude of other divers--no one, especially neophytes, want to look bad in public. just look at the judgmental statements that permeate discussions. no one ever criticized my choice of skis or what I wore on the slopes.
I have never heard that the training is too hard/too soft (maybe too long) except from veteran divers who think the people getting certified today have it way too easy because they did not have to do push ups wearing a scuba tank and wetsuit. In 30 years of diving, that is not a skill that I have found