Most Helpful Girl
Because even though they cover approximately the same amount of skin, they're completely different articles of clothing with very different connotations, and society views them in different ways. A bikini is intended to be worn in public, at least in the appropriate setting of a beach or pool. But undergarments are always intended to be worn beneath other clothing, and if a girl is in her undies it usually means she has taken her clothes off or is in the process of getting dressed, which are things that society sees as private and personal acts. The whole difference is whether or not they are meant to be seen. It may not make total sense, but who said humans were logical?