I really don't get this fetish.
Although maybe it's because I see the armpit as more just a connection between the arm and body than a "Part" of the body. it's also not really unique for women
It's not like like the breasts which are unique to the female body or the ass which is still generally shaped differently from a guy's due to the hips (Women has wider hype bones if you didn't know).
Although given that I think most armpit fetishists like to smell them (from the couple times it appears in manga) I think it's more a sub section of scent fetishists. But in that case I feel other scents would do just as well and the armpit is just one of the places on the body that most easily develops body odor.
Okay now that I fully wrote that out I guess it makes a little more sense. But the focus on pits alone is still odd to me.