Because the female body is an object of shame, and to be covered up unless viewed by mature adults. Duh. That's the same reason for every other double standard, like
1. Men who bang lots of women are studs, but women who bang lots of men are whores.
2. Strong men are manly, strong women are lesbians.
3. Men should enjoy sex a lot, but women who enjoy sex a lot either have serious issues, are whores or nymphomaniacs, or are just dirty
4. Men shouldn't have to wear condoms or pull out, that's inconvenient. Women should just take a magic pill (which screws with their hormones and body chemistry to a great degree).
5. If a man pays for a date, it means a woman should reciprocate with sex, because these are equal things.
6. If a man refuses a woman's advances, it's his prerogative. If a woman refuses a man's advances, she's just a cold *****.