Reduce damage to clothes
If the superhero is bulletproof, then it may be easier to make rip resistant clothing if it is close to the hero's skin. This allows the superhero's skin to stop the bullet as soon as possible after it hits the clothing. If the clothing were further from the skin, it might rip before the bullet gets a chance to create pressure on the skin.
To demonstrate, set up a paper towel such that it is stretched tight an inch in front of a board. Fire a BB at it. The BB probably goes through. Now move either the board or the paper towel such that they are touching. Fire the BB gun again. The paper towel likely stays intact.
This applies even more to making clothes abbreviated. Superheroines are often bulletproof or have accelerated healing. Clothes? Not so much. The sleeves and hose that you are not wearing are not in danger of getting ripped.
Freedom of movement
Put on a skin tight leotard. Try to move in it. Put on a woman's dress, try to move in that. In which would you prefer to fight for your life and those of your family, friends, and community?
You want your fighting clothes to move easily with you. Athletic clothing tends to be close fitting because that moves easily with you.
Easy to hide
Superheroes can wear their skintight suits under their clothes. That doesn't work as well if the suit is as bulky as the clothes.
She's a superheroine. Is she going to dress to hide herself? Or is she declaring that she is woman, hear her roar.
Many superheroines are overcoming shyness and timidity when in costume. Part of that may well be a reluctance to dress sexy. So when she is expressing herself, it's not just physical power but sexiness. Mousy in real life but sexually aggressive in costume. She may not even realize that she is doing it. She represses herself in real life, but she can let go when in costume. This includes hitting the idiots as well.
Supervillains are sexist pigs
Supervillains are evil right? So clearly they are exactly the kinds of people who objectify women and can be distracted by the fan service shots.
Ooh, boobs! Kick to head. Ooh, birdies! Passes out.
Hit 'em while they ogle, ladies. That's how the smart heroines do it.
Hide her secret identity
Flashing skin draws attention to the skin and away from the face. She doesn't have to give the "Eyes are up here" speech. She can simply travel in anonymity, as she won't show those parts of herself in her real world identity.
This may not work as well on women, particularly straight women. But presumably women will better understand why a superheroine might want to keep her identity secret. After all, men have enough trouble with smarter women. Where would we be if women didn't even need us to open jars?