said by PastTense:
Sprint is headquartered in the Kansas City area (Overland Park), so probably decided high quality service was essential in its headquarters city. To keep up with Sprint competitively the other carriers probably decided to upgrade.
After living in KC for years with Sprint service, I can tell you that historically they've not treated KC any better than any other city--
"What makes us omniscient? Have we a record of omniscience? ...If we can't persuade nations with comparable values of the merit of our cause, we'd better reexamine our reasoning."
-United States Secretary of Defense (1961-1968) Robert S. McNamara