The serious modellers want "useful" locos that would have been common on the line being modelled (or at least the line used for inspiration), which means a lot of Black Fives, 0-6-0 goods and similar workhorses. Most other buyers want something "interesting", which could be a nice livery, a big Pacific, or a one-off. The U class, being plentiful, a bit less flashy than an N15 or Schools, but with several nice liveries, fall somewhere in between. I suppose I'd buy one in malachite.