It's quite simple, they don't see each other as "brother against brother". That's the problem and I don't mean all of them but there is the ideology that they are guided and the others misguided or even infidels and that is worse to them than a Christian or a Jew. because to them it is rejection of truth after been given or seeing it. Instead of seeing each others as bro's/sisters it's infidels, rafidi's or whatever else.
People have become their own God in judging others.
6:159 (Asad) VERILY, as for those who have broken the unity of their faith and have become sects - thou hast nothing to do with them. Behold, their case rests with God: and in time He will make them understand what they were doing
42:15 (Asad) Because of this, then, summon [all mankind], and pursue the right course, as thou hast been bidden [by God]; and do not follow their likes and dislikes, but say: ?I believe in whatever revelation God has be?stowed from on high; and I am bidden to bring about equity in your mutual views. God is our Sustainer as well as your Sustainer. To us shall be accounted our deeds, and to you, your deeds. Let there be no contention between us and you: God will bring us all together - for with Him is all journeys? end.?
Peace