Primarily two reasons:

1. Over 78% of air is comprised of N2, while less than 0.1% is CO2. So, we have a lot more nitrogen readily available.
2. CO2 has a molar mass of 44g/mol, while N2 is significantly lighter at 28g/mol. Seeing as how everyone wants less unsprung mass, nitrogen is also the clear winner in this category as well -- those nitrogen tires will be about 36% lighter than the CO2 ones!

Of course, you can argue that since CO2 is a bigger and heavier molecule then it will leak more slowly from your tires (about 20% more slowly per Graham's Law!), but I believe that to be largely overshadowed by the two points mentioned above.

Primarily two reasons:

1. Over 78% of air is comprised of N2, while less than 0.1% is CO2. So, we have a lot more nitrogen readily available.
2. CO2 has a molar mass of 44g/mol, while N2 is significantly lighter at 28g/mol. Seeing as how everyone wants less unsprung mass, nitrogen is also the clear winner in this category as well -- those nitrogen tires will be about 36% lighter inside than the CO2 ones!

Of course, you can argue that since CO2 is a bigger and heavier molecule then it will leak more slowly from your tires (about 20% more slowly per Graham's Law!), but I believe that to be largely overshadowed by the two points mentioned above.