To answer you other question, I think most other American superheroes (white and non) are Christian (aside from some obvious of-course-nots like Thor and the Inhumans), it's just not ever really brought up, it's just assumed. Likewise, with the new Ms M, the fact that she's a Muslim is only mentioned in the first issue or two, and then isn't really a relevant factor.
But speaking to the larger issue of your question, I think it's because Marvel realizes that there are alot of young people in this country who don't really fit into the 'big-strong-straight-white-christian(assumed)-man' cookie-cutter mold most superheroes are cut out of. It's alot easier to relate to a role model when they're someone already culturally or ethnically like you.
Also, probably to fight some anti-Muslim sentiment that is known to brew in certain parts of the US.
Also, to maybe try and expand their market to another 1.6 billion members of the population.
Also because diversity is fun.
Actually, I have as little to do with modern society as possible. I may have a white skin, I may speak English but that's about all I want from society and that is an accident of genetics and of birth. I dropped out and do not really or particularly care for the values, opinions or moral codes assumed to be predominant. This does not mean that I am immoral or outside of the law - it means that I do no harm to others and when it is done to me, I choose to respond with the appropriate level of retaliation as is necessary and workable within my own moral and ethnical boundaries. I live by a complex code it could be argued but it is moral and honor is high up there and it is a tenant that I consider in every act I manifest. I think the lesson here is, white people speaking English may not necessarily be part of the white people, speaking English society. I have no issue whatsoever with a person because of their skin, their value systems or how their society suggests they might act. I do however have a great deal of issue with when such values are seemingly applied to me by others for whatever reason seems important to them - and I suspect I've become this way because it has been my experience that too many people on this planet seem obsessed with ordering others around; telling them what they can and cannot do. I appreciate that we need social order or we would have anarchy but are the wishes of the one, when forced on the many a form of dictatorship? You know, the old "do this, believe this - act this way or be considered an outsider!" type mentality.
Indian, Pakistani - what are political lines on a map in the cosmic scheme of things?
Why does 'religion'' even have to be a part of it? We have religion shoved in our faces on a daily basis and I find the whole subject highly offensive.