It could be a few reasons - years of waxing, tanning -
The exact cause is not known, but acanthosis nigricans is often seen in association with endocrine disorders such as insulin resistance and PCOS. This is a darkening of the skin ranging from tan to dark brown/black that commonly appears on the back of the neck, armpits, under breasts, in the groin area, and sometimes on the elbows, knees and hands. The skin is usually velvety or rough to the touch. It may look as if it is dirty and could be scrubbed off. Acanthosis nigricans cannot be cured, but individuals may see improvement if hormone balance is restored or weight loss is achieved. Some prescription medications such as Retin-A, 15% urea, alpha hydroxyacid, and salicylic acid may reduce discoloration.