They have a very elitist stance - if an author gets popular, no matter how "good" or "bad" he or she is, the prize seemingly cannot be awarded to them.
http://www.today.com/id/15210453/ns/today-today_books/t/nobe...
"A political bias may be hard to prove but there is no doubt the academy favors lesser-known writers to international best sellers. Few outside literary circles had heard of China’s Gao Xingjian, Poland’s Wislawa Szymborska or Kenzaburo Oe of Japan.
Gert Fylking, a Swedish radio journalist taunted the academy for a decade by showing up at the award announcement and yelling 'finally' when Engdahl read the winner’s name. After being barred from entering one year, he showed up the next in disguise.
'The point is that 99 percent of the time, the writer who wins the Nobel Prize is completely unknown to the big masses,' Fylking said."