Drawn mediums require by necessity that an unusual amount of focus be placed in human anatomy, particular in areas of the body that move, in order to portray that movement accurately. This means any area of the body that is both a locus for movement and high in visible muscle definition is likely to stand out both as a focal point for animation and as generally more detailed than than surrounding body parts that are less anatomically complicated to depict.
Waists, collarbones, armpits, hips, thighs etc are all things a viewer/observer's attention are drawn to because whether you're looking at animation or simply still illustrations, these are the essential points where the character's pose and expression are constructed. This is where body language is formed, and also where the most detail (second to the face/hair) is necessarily drawn in order to depict accurately.
Fetishism arises here from two things: the fact that these are focal points for expression leads to an association between their visibility and the things expressed when they are visible, and from the fact that, as part of human anatomy, they naturally associate with lewdness. The armpit suggests the shape of the torso leading to the waist, the leanness of the muscle underneath or the sexually dimorphic curves, the anchor of the brests along with their size and shape and way gravity affects them, the pose of the arms and what's being expressed through body language by that pose, etc.
It's no coincidence that many of these hyperspecific body-piece fetishes are often linked. You'll rarely see someone with a fetish exclusively for armpits, it's usually wrapped up in fetishes for hair, anal, navels, etc. because ultimately each piece suggests each other piece, and collectively they suggest a whole and simply serve as focal points of detail and motion to help construct that whole.