Because they are terrorists, they want terror. They dont want culture exept theirs but they don't follow the Quran, they are like a sect (Khawarij).
Quran: Surah 49 Verse 13 ''O mankind, indeed We have created you from male and female and made you peoples and tribes that you may know one another. Indeed, the most noble of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.''