Turkey has a very large military, easily capable of destroying ISIS. Perhaps the Russian involvement in Syria makes things more complicated, but that's relatively recent. Turkey was already spending huge amounts of money to house millions of Syrian refugees. I strongly suspect it would be cheaper for them to just stabilize Syria directly by military force, and send all those people home. So why don't they?
I'm guessing the answer has a lot to do with religion, pre-existing alliances, and balance of power in the middle east; if Turkey invades Syria, Iran and Saudi Arabia might object. But that's just a guess, and I don't have any clear idea of the motivations of Turkey in this situation vis a vis the other major powers in the region.