Why? Because it would be too disruptive to his or her understanding of the world to discover that Islam — a "brown people's religion" in that worldview — is a dangerous ideology and the cause of much of the misery in the world today (and throughout history).
So don't be surprised at the intensity of the resistance you get when you're talking about simple facts about Islam, Muhammad, Islamic history, etc. The resistance is so intense because this is not a simple process of updating facts about something relatively unimportant, like honeydew melon. "Oh, they're high in carotene? I thought cantaloupe was the only melon that was high in carotene."
People update their information easily and with very little resistance — unless it challenges something important they believe. And if it challenges a cornerstone of their worldview, the resistance you get is surprisingly forceful. It has the feeling of desperate ferocity. They will argue with you about Islam, sometimes quite passionately, even if they know little about it. They might not know why, but they do NOT want what you're saying to be true. So they'll come up with every argument they can think of, and if that doesn't work, they'll try to write you off as a ________ (fill in the blank: bigot, racist, hater, Islamophobe, etc., something that allows them to disregard your information).
But it doesn't work. Say it anyway. It DOES sink in eventually. It just takes awhile sometimes because it alters something very fundamental about their worldview, and that takes some internal adjustment. Don't worry about it. Inform them — gently if you can, and make it as interesting as possible — but don't stop trying to get through to people. It's the first thing we need to accomplish. Here's why.