While by most accounts GT6 is solid, GT5 was such a disaster I think that GT6 deserves poor sales. People that build up fanbases of millions of people shouldn't knowingly screw them and Sony and PD knowingly screwed fans when they shipped that broken POS.
The numbers are most striking when comparing US sales figures. In December, analysis firm Cowen & Co estimates that GT6 sold 300,000 units stateside. Over the same period, GT5 sold 1,169,074 copies to American owners.