Concrete curing is a chemical reaction that's sensitive to temperature. Pour in weather that's too cold and the reaction slows way down, which can weaken the surface or cause it to crack early. Pour in extreme heat and the surface can dry out faster than the concrete underneath cures, causing cracking from the other direction.
That gives Denver Metro a fairly wide, but not unlimited, window. Late spring through early fall generally offers the most stable temperatures for a clean pour and predictable cure. We can still pour outside that window with cold-weather concrete mixes and protective blankets if timing doesn't allow for waiting, but it adds cost and complexity.
If you're planning a driveway replacement, the practical advice is: don't wait until the ground is already frozen to call. Book far enough ahead that the pour lands in a stable weather window, especially since a lot of homeowners are trying to schedule projects in the same few months.