Yeah I guess nobody really takes pictures with the iPad and so the cost factor is not worth the investment.To lower production costs and higher profit margins?
I think the fact of it is, is that people are much more likely to be taking pictures with their iPhone 4s than with their 2012 iPad. So thusly, better camera for where you need it, slightly worse camera for where you don't need it.
Besides, megapixels aren't everything. Maybe the iPad's sensor is larger and better. Who knows
EDIT: darn, number84 beat me to it!