Reasons that I hear from my friends and coworkers are these:
1) Hated cold water. I know a number of people that got certified but hated the cold. Many did some tropical dives, and that was the end of cold water diving. I have friends that used to go twice a year to warm waters, but the economic downturn has squelched that, so they haven't gotten wet in a few years. Heck, we know a couple in Florida where the wife won't dive in winter because it is too cold. The water was 84 degrees for goodness sakes, and she said it was too cold!
2) No reliable dive buddies. It is difficult to stay with it when you don't have anyone to share the experiences with. Once you fall out of the habit of diving occasionally, you drift away. I know a LOT of people that "used to dive a long time ago".
3) Other interests and pursuits. This includes SO's and families. A good friend and his wife loved to dive, but starting a family has killed it. A coworker used to dive a LOT, but three kids later, he still gets "wet" but not by diving.
4) Cost -vs- Reward. It gets harder to justify buying the gear if you only dive once a year on vacation, so they just don't stay with it.
5) Hard to restart. Once they stop for a while, it becomes very hard to get started again. One buddy broke an ankle playing hockey, and that forced a stop to diving also. He restarted playing hockey, but he hasn't been underwater in a couple years.
Just my data points from friends, coworkers and club members.
Wristshot