Muslim women are only allowed to expose their face and hands according to the traditional understanding, and even the face is a lot according to some who order women to cover the cover the face as well. Many sheikhs become angry if a few strand of hair get exposed.
But Christian women don’t have such strict, rigid dress code for them, as you can see in the right picture, dress like this where they don’t cover their arms, legs, head, neck would be accepted as modest by their Churches. People on this subreddit won’t find any immodesty in them either, but a lot of Mainstream Muslims surely do.
What is the reason?
I’m an atheist, but What you consider “mainstream” is a little bit wrong in my opinion. I don’t know if there’s been a world wide study on the percentage of women in each religion that wear what we would call restrictive, but it isn’t that different. Indonesia has the largest Muslim population in the world and when you think of Indonesian women restrictive clothing and way of life, doesn’t come across as more draconian than devout Catholic women in my Hispanic area. I think this is more a case of which group gets more attention in our bubbles. Western nations just can’t stop focusing on what’s going on in Muslim dominated areas because of oil and holy locations. Casting them as somehow backwards is how we justify bombings and occupations.
Edit: Also do we just ignore how Orthodox women be dressing?