The neighbouring regional powers, especially Turkey, with their own sizeable and restive Kurdish populations, won't permit it. Moreover, the landlocked Kurdish territories are dependent on them for gas and oil exports and for imports.
"We don’t want to be used as cannon fodder to take Raqqa,’ a Syrian Kurdish leader in Rojava told Cockburn. "Once Mosul is liberated and IS defeated, the Kurds won’t have the same value internationally", a left-wing Iraqi peshmerga commander added ruefully.
They were doubtlessly referring to their military value to the US in the war against ISIS. Whether the Kurds' isolation in the region and the US interest in maintaining a foothold in the Middle East other than Israel continues to draw the two sides closer together remains to be seen.
http://www.lrb.co.uk/v38/n05/patrick-cockburn/end-times-for-the-caliphate