The sweat glands on the head (and most of the body) differ from the sweat glands in the groin (and armpits, eyelids, ear canal, etc). The kind of sweat glands found in the nether region excrete fat and protein, feeding bacteria which causes the unique odor of those regions.

Also, those regions get less air due to clothing so are better habitats for bacteria.

Also, of course, your butt and urination device are the trash bins/shoots of your body and are home to more pathogens and parasites.