Why are there so many ADS-B ground towers in western Colorado? I was reading up on Appaero's new ADS-B OUT transponder and clicked over to the Stratus coverage page for some reason. Then I glanced at the map, and noticed something interesting:



Western Colorado is, well, ADS-B tower central. What gives? Chicago and the northeast are close, but not nearly as dense.
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.