ISIS says they adhere to Islam, they cite the Koran, they want to establish a global caliphate under sharia law, they are not affiliated with any other religion except Islam.  No-one else, either other Muslims or you, gets to say they're not Muslim - they say they are so they are.  You don't get to say just what's pleasant to your ears, that to be part of a religion you have to be nice to people, that's a total lack of logic.  In answer to your question, that's why.