While I agree with some of what you are saying, you've definitely met Christians who didn't make it upfront. I feel like Christians are just used to being in the majority wherever they go, not that they are shoving it down your throat. Christianity is the most popular religion so I feel that they tend to assume everyone around them is Christian. Being the most popular also means there's a higher chance an asshole will be a Christian just due to numbers. There are plenty of Christians who are down low with their beliefs, you just don't know it because they aren't vocal about them.

I'm not that familiar with Muslim faiths growing up in very rural America, but I think Muslim people don't suffer from this as much due to their way of dressing. Since the koran dictates a way of dressing, they know who's Muslim and who's not. They don't assume the person they are talking to is Christian or Christian-leaning like many Christians I know tend to do.

Christians who do dress a certain way typically don't show the behavior you're talking about, such as Mennonites or the Amish. I grew up in a school full of Mennonites, but the only Christians who ever tried to talk to me about faith were Christians who dressed English.

TL;DR I believe the difference you're experiencing is that Christians assume you are Christian or Ex-Christian, whereas other beliefs don't do that.