The current ICBM force consists of Minuteman III missiles located at the 90th Missile Wing at F.E. Warren Air Force Base, Wyoming; the 341st Missile Wing at Malmstrom Air Force Base, Montana; and the 91st Missile Wing at Minot Air Force Base, North Dakota.
Not upgrading the missile fleet means the end of these bases, and the jobs that come with them in areas where they represent a huge piece of the economy.
"Malmstrom alone is one of the region’s largest employers. The base had an economic impact of $372 million in 2019, including direct and indirect jobs as well as expenditures on construction and other services."
Local politics and economics hold more weight than strategic and technical sense. Try closing a base and getting re-elected in today's USA.