Why hair pins are called bobby pins?

The style of hair pins (not to be confused with hairpin, another style of coiffure pin) known as bobby pins in the US are called so after the trademarked name they were originally marketed under, now out of trademark. As the company is called Bob Lépine, it's safe to say the name comes from the company name (probably the founder of the company).

From Wikipedia - "A trademark on the term "bobby pin" was held for some decades by Bob Lépine Corporation of Buffalo, New York."

Similarly, the British name for the same article is "kirby grip," and is derived from the trademarkKirbigrip, used by a Birmingham manufacturer of such pins, Kirby, Beard & Co. Ltd.