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?
Most common reasoning for any religious related question is “it was this way for centuries so it should be this way further” or “main book demands/explains it this way”.
Also major crisis usually makes people return to fundamentalist views. Many Islam based societies were well on their way to reform and improvement, but then the west kinda went and bombed the shit out of them to get their natural resources. I think its fair to largely attribute stagnation in Islams progress towards womens rights to the wars and bombings funded by the US and Russia.
That doesnt excuse their treatment of women, but it might explain it to a degree. The more peaceful and fair the environment, the less religeous people become. The less religeous people become, the better they treat women and people of other beliefs.