Obama is damned if he does, and damned if he doesn't.
If he does send in troops, he knows he'll be getting in a war and letting his successor do most of the heavy lifting. In addition, there'd be the issue of financing of the war and the loss of other funding if he goes through with it. He'd have to sacrifice immigration, Obamacare, planned parenthood and maybe more just to justify the financial cost of war. Then comes the issue of the killing of civilians which are bound to happen as Daesh uses Yazidis as human shields as well as bombing mistakes, drone mistakes, etc. Then, to top it all off, this is exactly what Daesh wants as it claims the end of the world will happen once the forces of the West meet it at Dabiq and Daesh supposedly routs it. With an invasion come the rise of people against the West who join ISIS solely for combat. In essence, it becomes an ever-larger shit-storm set off for revenge, with troops dying, innocents dying, global condemnation of America's "imperialist" aims, and so on and so forth.
If he doesn't, well, you see it here. Obama gets called a coward, and jingoism starts emerging in full force. It's a lose-lose for Obama, and he's picking the side where he loses least.