Several reasons. Designed to use concentric coils and many prefer DD's. Concentrics offer better pinpointing plus better depth on pasture but on heavily contaminated ploughed fields DD is the way to go.
They also don't like light dry, fluffy soil.
Concentrics target mask more.
Wet beach performance is poor.
Cheaper detectors came along that offer good performance, almost in a switch on and go package. If your not going to use the various settings of the XLT/Spectrum's whose inbuilt programmes are frankly pretty poor then its better to take the simple route. The programmes can't really be optimised by Whites as they have to make them to work in all the hundreds of countries they export to. They are a starting point on which you have to build.
Then there's the fact that the DFX came along that solved the wet sand problems and didn't really cost anymore to build (apart from paying Minelab).