This is a reason why Pop Ups aren't selling......

I'd say expense and weight. The two main reasons people opt for a popup is low weight for easy towing and lower costs compared to a TT.

Todays popups are NOT light weight and NOT low cost. In the early 90's we bought a Coleman Roanoke. Weighed about 1,000lbs fully loaded. I towed it for years with a lowly Suzuki Samurai. Today's lightest Jayco popup is 1500lbs dry and priced more than three times as much. Inflation doesn't account for the price.