Big Brain is a great game for people who enjoy puzzles and games that make you think. Everyone that I know that has played either the DS version or the Wii version loves the game. Maybe it's the demographic that my friends and I fit in that explains why we love it. One thing I do know is that the DS version is very addictive to me and I can't wait to get my Wii back so I can play Wii Degree.
It's probably just not a genre that you are drawn too, just like I could care less for the gore games like Manhunt. There must be a huge market for these games b/c Big Brain and Brain Age continue to sell.