Salaam Aleykum,
We have a thread dedicated to asking Questions about Christianity and recieving replies from Christians.
Moving on, you have three questions and not one.
1. Why are some Christians who support the choice of clothing of Nuns not supportive of the Muslimahs choice.
The answer from my view point is that many people see the choice of Nuns outfit as a personal choice of that Nun, so they see that the Nun choses to cover herself in a manner, which the people may believe is good, but at the same time the people may not see it as an obligation on themselvs.
It becomes clear then that some Christians see the Nuns clothing as a personal choice.
On the other hand some Christians, non-Muslims and Muslims, have the idea that Muslimahs are forced by their husbands/fathers/Imams, to cover in the style that they do, from this some people understand or assume that some Muslimahs do not want to cover but are being oppressed and are scared to tell their husbands/fathers/Imams that they do not want to cover.
So we see that on the other hand some people see what Muslimahs wear as not a choice of their own but something emposed by people around them.
2.Why can't other Christians follow the example of the Nuns.
Firstly it could be that other Christians do not feel the same about this, Nuns are Women who have devoted their whole life to G-d in the sense that they will not marry and remain virgins, and so forth. Not all Christians want to do this.
Also over the years some Christians have 'intergrated' with 'fashion' and do not see anything wrong with wearing short skirts and bikinis, as long as they are not sinning.
So we see wearing what Nuns wear is not an obligation, thus not all chose to do it.
3.This is the same as the previous in the sense that, again it is not an obligation, the equivilant is that of wearing an act of sunnah in Islam.
It is not fard for a Christian to wear a cap, rather it maybe seen as something good to do. Though I have not seen no Biblical Evidence for wearing a cap.
