For the same reason that Communism and Socialism are still hated, associating a single attribute to 'the enemy' and having them become synonymous to a point where the attribute itself is infamous for no good reason.
The problem is not Islam, or Christianity, the problem lies in the fact that religion has lost its spirituality, it has evolved into a political tool, which is directed at the manipulation of human consciousness.
These days, some Americans are spoon fed with half truths and stupidity that its becoming more of a norm to not use rational thinking. But out of that bunch, sometimes they are right to fear and hate Islam for a reason. I was one of them. Its a terrible mindset on what we see on something foreign, that's our nature. The reality is that when someone gets killed, assaulted, or discriminated by a muslim, of course people will look upon that man/woman and judge their faith. That's how it was back then, that's how it is today. There is no greater evil but ignorance.
At least I was fortunate to have a history teacher to taught me how to understand the world. If I was you, I wouldn't be bothered too much about the mindset of Americans, for its not your lost. Stay truth to your faith and go on with your life.