The Meier's Decision in Federal court evidently accorded Denver's exception due to their ordinance having been in effect -before- the Colorado Constitution was crafted. Established as a "home rule" city, and having their firearms ordinance in place prior to the existence of the language of the Constitution, their firearms language was "grandfathered" in as exempt from the preemption law. When argued in court, the ruling was based on whether the State of Colorado had a significant interest as related to preemption in this case, to overrule Denver's ordinance. Denver argued they are an urban municipality where, due to the density of population, it was in their best interest that their firearms restriction be upheld. Oddly enough, I didn't see anywhere that indicated why the urban nature of Denver made any difference between a handgun carried concaled with a permit or one openly carried.
Anyhow, I haven't explored the subject fully, and what I provide above is my impression of the basics of the case as expressed in the Meier's decision. If you'd like to learn more, you can find a sticky at the top of the Colorado board that has all the relevant facts of the case,
Peterson v. LaCabe (Denver, CO) MSJ Filed
and can give you the language of the actual decision handed down in federal court.
Interestingly enough, the state capitol is located smack in the middle of Denver.
Maybe we should start a movement to move the state capitol out of Denver to a city that upholds the Constitution of the State of Colorado, like Colorado Springs? Or even Aurora? If Denver does not wish to adhere to the Constitution of the State of Colorado, maybe the State of Colorado should move to a city that does!