Someone will no doubt correct my hazy memory, but I wonder how much to blame was the fact that default QT apps were so ugly around the time that was the height of RAD, and QT was the only(?) well-supported cross-platform cross-DE gui framework at that time.

On windows you could RAD your way to a good looking windows application that looked and behaved like everything else. On linux, default behaviour was either ugly, or not very compatible, or heavily customised anyway, and that didn't change until much later with improvements to GTK.