Why are girls embarrased in a bra and underwear yet eager to be seen in an even more revealing bikini?

Consent. If she's on her bikini and she's outside that means she wants to show off her body and wants to be seen. If you catch a girl in her bra and underwear and she's embarrassed then she did not want to be seen. Which also means you violated her personal space at the time she did not want to be seen.