Because most of the music they make these days is shit, society is in perpetual decline, the days are long gone when you could buy anything you wanted for tuppence and still receive change and youngsters don't believe me when I tell them I grew up in a cardboard box on the highway. Graham Bonnet deserves a mention in this thread. So does Dio. Bruce Dickinson, of course. Gayer than a French sailor, but it would be blasphemy to create a thread like this and forget about Rob Halford.
There are many great contemporary singers, simply not a very common breed in popular music simply because they actually try to sing rather than try to make something catchy to sell