Nose got used to it
A person's brain is more sensitive to "changes" in aroma and can "ignore" odors once he's smelled them for a while.
Halitosis occurs for several reasons. Chemicals from some foods/drinks diffuse into the flesh in the mouth and throat. These chemicals (odors) keep coming out into the breath for many hours. Some foods/drinks cause odors when in the stomach. Some substances stick to the teeth.