Why Sunni Governments in the Middle East aren't fighting ISIS much?

They don't see ISIS as the worst threat to their existence. The worst threat is Iran, Assad and Russia in combination.

Though ISIS wants to overthrow all their governments, ISIS doesn't want to overthrow Sunni nations as much as all European nations and the U.S. and Canada. So, it is much more in the U.S. and Europe's and Canada's interests to get rid of ISIS than it is for Sunni nations like Turkey, Saudi Arabia, Jordan and other Sunni Governments in the Middle East.

So, about 95% of the military resources applied against ISIS are from the Shia National Governments of Iraq and Syria (even though the majority in Syria are Sunni). And from the U.S., Canada and Europe.

As a result of Shia Muslims fighting ISIS and Europe and the U.S. and Canada fighting ISIS, it is likely that the Islamic State will be no more within a year or two. IN other words the next U.S. president will be able to say, "The Islamic State is gone along with all it's armies and tanks and cannons." However, he or she will not be able to say, "ISIS is gone as a terrorist organization."

Another point of view is that Russia's interests don't include ending ISIS completely. The reason for this is ISIS destabilizes Europe. So, to some degree as refugees from Syria and Iraq and Libya destabilize Europe like through Brexit and other problems, Russia benefits from the weakening of Europe.

As long as Russia can keep their warmwater port in Tartus in Syria and can keep Assad in power in Syria they are going to be happy watching Europe and the European Union destroyed by refugees coming into Europe from Syria and Iraq and Libya.