Because Islam is not a unified entity. You don't have to be paying very close attention at all to have noticed that Shia Muslims always seem to be in conflict with Sunni Muslims. It doesn't take excruciatingly long and difficult research to discover that the Shia-Sunni split goes back to within a few decades of Muhammed's death. Muslims haven't all agreed about anything since the 7th century. If you've gone from a horrified rejection of any examination of Islam to a casual interest in it, you can also see that even though one individual is a Shiite and another is a Sunni, they often will get along just fine. The way there's nothing at all strange about a Catholic and a Protestant being friends.
The same way that there's nothing at all strange about a Catholic and a Protestant and an Orthodox Christian and a Copt and a Shiite and a Sunni and a Hindu and a Sikh and a Buddhist and a Druze and a Bahá'í and an atheist all being good friends with each other and getting along just fine, although many of those individuals, perhaps even every single one of such a circle of friends, might unfortunately know people who share his or her religion or lack of it and think that it's not fine at all to be mingling with those others. But most people in each one of those groups don't want to kill over the differences between the groups.
I'd like to think that the great majority of people belonging to each one of those religious or non-religious groups feels a stronger identification with the entire group of humans than with their own individual group, and thinks of the people in the other groups as people like them, who get happy and sad and hungry and sleepy and have friends and families and quirks and senses of humor and hopes about putting ethnic and religious hatreds behind us once and for all, the sooner the better!
Let's compare the tensions over religion which are obsessing some people so much these days with ethnic tensions. There have been some terrible atrocities committed against non-white people by white people, horrible deeds committed in the name of white people. Nevertheless, white people are not a hate group. Why? Because the people who have done those horrible things, although they claimed to be acting on behalf of all white people, did not have the approval or backing of all white people. In the case of the Nazis in WWII, many of the people, probably most of the people, who fought against them and finally defeated them were white people. You didn't have to be non-white to vehemently hate everything the Nazis stood for.
In just the same way, most of the people fighting against ISIS today are Muslims. That's no secret, there hasn't been any official media black-out of that information in the West as far as I can see, and yet, many people act as if it would be news to them. As if Islam were one unified group which was coming to get us. The same way that some Muslims -- ISIS, for example -- talk about the West as if the West were one unified group which was coming to get them.
Imagine if in the midst of the Thirty Years' War, some Muslims in Egypt or Persia talked about how the Christians thought of nothing day and night except how they were going to crush Islam. Some Christian idiots were pre- occupied with such thoughts then, just as some Christian and New Atheist idiots think of little else today. Others were much too busy with the mostly Catholic-against-Protestant conflict in Europe to give much thought to Muslims.
Then as now, the biggest problem facing humanity was not one religious or ethnic group against another, but many different groups of idiots, large groups and smaller ones, each group against pretty much the entire rest of the world.