I had thought it was because they were satisfied with the intercity service provided by Colorado Motorway, at the time of RTD’s inception. The James family sold their Denver Boulder Bus Company to RTD, and shortly thereafter found a buyer for their CMW with Continental Trailways, which already ran a few trips through many of the same towns on its Denver-Wyoming routes, albeit with an intrastate traffic restriction. When Continental Trailways took it over, they began cutting service, and when it was acquired by Greyhound, even more cuts.Larimer County's non-interest in joining RTD was because it already had a muni system in Fort Collins and subsequently one was set up in Loveland
Today, Greyhound is down to just a single trip that way, that just serves Fort Collins on its way across Wyoming to Salt Lake City.
“Bustang” does provide multiple trips with some stops between Denver and Fort Collins…