a) the shear scale of their email operation = large costs
b) poor conversion on those ads as users are not in the "search" mindset when reading emails where as they are more likely to click on an ad when searching
This is from someone I know who works in/around GMail team at Google. I have also seen research from the perspective of advertisers as to the performance of their ads on SERP pages verses appearing in GMail - I can dig that out if you are interested, let me know.
a) the shear scale of their email operation = large costs b) poor conversion on those ads as users are not in the "search" mindset when reading emails where as they are more likely to click on an ad when searching