Why are they cheap? £512pa VED, fuel consumption, insurance, and the fact they're a little understated. Then you've got the largley unfounded reliability concerns. Plus they're not necessarily the best cars in their class in absolute terms, and they have to compete in the used market with M5s and fast Merc saloons, which are numerous and cheap. On the plus side they're excellent, largely reliable cars with a lovely-sounding V8.
The QP V is probably the best looking four door saloon ever made with a soundtrack to die for and I was a proud owner of a facelift a year ago...
However... to the average petrolhead there are two main problems -
1. Power; at the QPV's price point there is so many other options with circa 100bhp more (M5/M6, E63, RS6, XFR)
2. Perceived poor reliability and exorbitant running costs; German “reliability†has been drilled into the nation for that long coupled with the Maserati (general Italian?) image of old makes the perfect storm where potential owners don't even put a QPV on their shortlist. Throw in the fact that their parts aren't of a shared platform (an E63 is pretty much a bog standard E-Class bar the engine, brakes and suspension) and you've got a minuscule potential customer base.
I'm looking to buy a W221 S65 AMG in the new year but I do keep coming back to the Quattroporte pretty much because of the bang for buck you get; everyone I came across in my old one loved it and lost count of the amount of positive comments I got.