Why did western territories take longer than eastern territories to achieve statehood?

Statehood could have been achieved sooner for the states in the western territory if it had not been for an extraordinary set of conditions linking to political considerations on the national scene and long-lasting internal strife among those who lived in the state in the western territory. 

yes, there are senators in each state

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.