Diddy Kong Racing sold almost 5 million units (4.88m). It seems unbelievable that Nintendo the king of milking hasn't released a sequel (Diddy Kong Racing DS doesn't count) yet.
If Donkey Kong Country: Returns has managed to sell over 4 million units it sounds logical that an hypothetical Diddy Kong Racing Wii would have sold even more since Mario Kart Wii has established a huge fanbase for this kind of games.
Do we have other examples of huge selling games that never got a sequel?