Why can't we smell our own bad breath?
What you might be experiencing is olfactory fatigue, the temporary, normal inability to distinguish a particular odor after a prolonged exposure to it. 

It's a form of neural adaptation, a change over time in the responsiveness of the sensory system to a constant stimulus. This adaptative mechanism might prevent the overloading of the nervous system.

However, some people (maybe most of them?) are able to smell their own breath. At least, I know I do. I can't point you to any statistics, but it might be an interesting research project, figuring out how many people can smell their own breath.