Why is the atomic clock in Colorado?
First of all, there is no "the atomic clock," the timing signal
your receive is actually comprised of data from many atomic clocks.
A radio signal is generated by NIST in Colorado, partly because a
portion of NIST is located in Colorado. Having a more-or-less
centrally located radio source maximizes radio coverage in the
continental US. There is also an atomic clock ensemble at the US
Naval Observatory in Washington DC, which is the timing source for
the Department of Defense.