Why is this happening?

From here on in, I will only refer to ISIS as Al-Qaeda separatists. They are not 'Islamic', and they are not a 'State', and they certainly do not belong to Iraq or the Levant. I will not collude with their delusions of grandeur. They are a bunch of criminals so depraved, that even Al-Qaeda has distanced themselves from them.
They are an international hodge podge of power hungry, arrogant, criminals, united in the search for money, power and degradation and mass murder of civilians. They use religion as a smoke screen, a red herring to pull ordinary folk off the scent of their real intent. They have nothing to do with faith. The longer the general public is fooled by this, the longer it will take to deal with the real problem.

Their 'fighters' get paid at least double the monthly wage of an Iraqi soldier. Where is this money coming from?

They have weapons, that some states can not buy. Where did they get the money? Why is the arms industry selling to them?

They have actively sought to attack, take over, then somehow continue to run gas and oil fields. They have taken over most of the gas and oil plants in Syria, and at least 5 in northern Iraq. How are these plants still running (random untrained crazies are not going to be capable of this)? Who is buying the oil?
Interestingly, the price of oil internationally has gone down corresponding with the above take overs of natural resources. There are suggestions that this oil is being sold cut price, going over the border to Turkey, and being sold on to progressively bigger corporations.
Buying this oil is effectively fuelling the conflict. Why is there not an international response to ban the buying of illegally sourced oil by the corporations? Why is there not an international boycott to these murky practices that fund this international criminal activity?

This is about money, power and degradation. This has never been about religion. There is an element of cult like practices in some segments, and atrocious lack of rational thought, but this isn't the bigger picture, they are not the big fish. The longer we are fooled by their rhetoric, and believe their narrative, the longer this will go on, and the worse this will get.

They are an international mafia like corporation.
A lesson from history - Al Capone wasn't put away for murder. He was put away for tax evasion.
At this stage, we don't really know who the real 'Al Capones' are, but the income streams are increasingly clear.

https://uk.news.yahoo.com/video/release-video-bodies-captured-soldiers-083038368.html