I read an excellent and pretty exhaustive article about Saudi Arabia - ISIL relationship. It's a very complex and centuries long story between Saudi monarchs and extreme Wahabbi movements, movements whom ISIL declare as forefathers. Currently, KSA is starting to deploy a "wall" on Iraq border (against ISIL). Wall is quoted because it's last iteration of Berlin wall times 10. A defense strip with multiple layers of trenches, barbed wire, walls, defense towers, radars, etc...
Because ISIL also wants Mecca wiped. They hate the Black Stone particularly, for them it's an ultimate symbol/material worship and it's a no-no.
Maybe that's why world powers had a mellow reaction on the whole "caliphate" business. ISIL by definition wants almost everyone dead, now they're turning against their sponsors. Perhaps everyone is waiting for that point where a hypothetical Middle East coalition just wipes out ISIL because they surrounded themselves with enemies. If they lose mobility over their southern borders, they're effectively done for in a matter of 6 months.