You seriously don't want the saudis and their own whack job version of fundamentalist islam to get militarily active in more places in the Middle East.
... and Turkey is making huge profits from buying black market oil from IS and probably pissed at Russians meddling in that by bombing oil convoys ... it's a labyrinthine mess...
In theory ISIS should be easy to choke off, as it has no industrial base -- but somebody's buying their oil, selling them weapons etc etc...