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?
Fundamentalism. If you look at photos of women in Iran in the 1970s, they look like the photo on the right for example.
There is currently a fundamentalist Christianity movement pushing for subservient women in the US.