Why are there so many ADS-B ground towers in western Colorado?
As GdD commented, the cluster you noticed is not near Denver, but Montrose, CO. The reason for this cluster is Wide Area Multilateration (WAM) with ADS-B. This technique uses many sensors distributed around an area to provide better coverage in areas with mountainous terrain that blocks a single radar or ADS-B station. Western Colorado is one such area that experiences delays when ATC does not have sufficient radar coverage to support operations in inclement weather. Montrose is one of the areas that the FAA is using to develop this technology. This "constellation" serves the following airports:


Gunnison-Crested Butte Regional (GUC)
Montrose Regional (MTJ)
Telluride Regional (TEX)
Durango-La Plata County (DRO)


See also this technical presentation about the WAM project, and the lease for the tower sites.

Also, somewhat related, see this NTSB safety recommendation regarding ADS-B coverage serving SAR needs after a crash near Montrose.