Probably because every large scale invasion in the region has ended in disaster. We can't go in without a plan for what to do after we finish bombing since these people don't wage symmetrical wars. Ultimately the problem will need to be solved by the locals. We can't force solutions on them.

First of all, consider how hard it's been to root out the Taliban in Afghanistan. Having technological superiority doesn't mean you can root out an army that has local support and has trained under the exact local conditions you're facing. At best you force them underground and then you face a guerrilla war of occupation in a foreign land. (See also Iraq.)

Second, ISIS might be demonized in the media, but from a geopolitical standpoint, it's useful to some groups. It may or may not be a proxy of Saudi Arabia in the region, for one. Second, they're undermining Syrian power and preventing al-Assad from consolidating his hold on the country. Together with al-Assad, they form a status quo that is better than either side winning. Finally, Turkey is quite content with letting them run amok south of the Turkish border, because those who are getting the brunt of their assault is the Kurds, whom the Turks categorically oppose.

Third point, ISIS is a useful boogeyman. Canada and the US like to claim that ISIS is a direct threat, but this is clearly bullshit. They're a threat to regional stability, but the link between ISIS and terrorist attacks abroad is debatable. However, it's convenient for governments like Canada's who want to toughen up surveillance laws. So you have a group whose existence is convenient, but who poses no actual threat. Would you take that out?

The truth is, the propaganda around ISIS is useful to both ISIS and Western governments. That's why you'll see, say, the Canadian government straight up use ISIS propaganda images: they're useful to both. But the realpolitik of taking out ISIS is way more complicated. So you end up with a lot of posturing and very little actually done.

I think the problem is that you can't wipe out an ideology like you can a rebel group. I wish it was that easy but it has to be handled carefully or it'll just breed !ore people like them.

Because war is massively economically and politically expensive. No global leader wants to spend political capital on people who won't vote for them. ISIS will thrive and expand until it is a direct threat. Only then will action be taken.

It's a lot harder than it seems.

As in Vietnam we can't fight this war in a conventional manner and the ultra conservation, tradition bound military can't seem to get their minds around this. Our idiotic invasion of Iraq created ISIS just as the allies' treatment of Germany after WWI created Hitler. Invasion and occupations, no matter how they are intended, cause dead people, destruction and resentment. In defeating an enemy we create new ones. The people of the middle east don't believe we're in this to help them. They know we're doing it for the oil and to keep corporations happy. Wars make a lot of people a lot of money. Just a preliminary reading of world history will show you that the third world has no reason to believe the west about anything and would be stupid if they did.