I know that everyone seem to hunt muley early but why not later season around 10000 feet pro&cons this hunt would be during the rifle with bows

Because that's during elk season! (grin)

 ElkNut1

By late sept the bucks have shed their velet. Having the soft skin on their horns keeps the bucks out in the open more. Once it comes off however, the bucks hit the timber and are VERY hard to find. Generally they don't stay up high in the open thta late, at least from my observations. Another factor is pressure. By that late in the season, the deer have seen a lot of guys chasing them. And there are not many seasons that run that late into Sept in the high country.
Those are just my observations though.

Wyoming for example, most mulie seasons are closed later. Most areas across the state have a September bow season, then a ten day gun season in Oct. Our G & F does not want us to hunt mulies late because they think we will kill ALL the big bucks.

As far as hunting late at 10,000 feet, there could be a heck of a bunch of snow up there. You may not even make it up there, and the deer may be down anyway.