being that in your profile you didn't post your location I can't say that the following reason applies to your location. But here is a major reason in our area:
I just recently again, heard of people that went on some southern caribbean trip (read: clear and warm water), got mesmerized by the snorkeling, took a weekend certification course, came back to the mostly dark, cold, and silty waters of home (mind you not all the diving is like that in Minnesota) all excited to dive again but decided if they never dove again.. so be it.
Well, here is what I offer up as a solution. Down in South Dakota, along the eastern border, they have some deep quarries, the result of mining granite out. Fill 'em up with salt water, place a solar dome of some sort over the top. Bring in all kinds of tropical fish and plant life. Get a circulation pump going, and a huge water heater.. warm that water and dome up to tropical climate. Place some palm trees in strategic locations, a little palapa providing your favorite beverages, etc. You'll have divers from all over the mid-upper west U.S. (maybe even the world) clammering to come a divin'.
I'm sure the fall out rate would drop dramatically if the dive industry would just provide more pleasurable diving conditions/locations a little closer to home.