Because it isn't about religion, it's about power. Islamists want power, not for everyone to respect religion the right way.
So when non-islamists, "moderates" or just another branch of islam, because you have Shia and Sunni yes, but you also have the Wahabi Sunnis(Saudi Arabia), the somewhat more moderate Egyptian Sunnis, the even more moderate Syrian and Lebanese Sunnis, etc.
All these different practices stem from different seats of power. Syria and its sort of adopted child Lebanon are friendly towards Iran and the Shia, and do not seek religious fascism.
Egypt has a much more radical view on religion, but avoids exporting it and seeks a relatively large intellectual and spiritual life.
Saudi Arabia practices the worst kind of islam, the very very medieval one, and intends for this kind of islam to be imposed worldwide with no regard towards what others might have to say about it.
ISIS is of course the bastard child of the US destruction of Irak(which created a power vacuum and thus room for a new power) and the massive support of Saudi Arabia towards the most radical elements of Islam.
On an ideological level, Egypt battles Saudi Arabia for influence, but is a ruined state and thus cannot battle the Swims-In-Oil nation. Syria is more mild, it's a smaller nation led by a dictator from a non-muslim minority, and thus tries to keep a balance by opening itself to Iran, and might be the only Sunni nation to do so.
To cut to the chase, the problem is and has always been Saudi Arabia in that matter, both because of their endless pits of black gold, and because of their position as an archaic absolute monarchy that sits on the most holy muslim sites and always had a large share of the muslim world's fanatics.
ISIS is mainly supported both in men and money by the Saudis, and they're the ones killing most muslims, except when the Statesians start bombing or pushing for revolution.