It's not as easy to understand as I'd like. Is the issue noise? Maybe, hard to tell? If it's not noise, a ban in parks seems very excessive. If it is noise, why is there no threshold you could try to be under?
From talking to a ranger here the issue in Boulder's parks (I don't know about elsewhere) is that they disturb nesting raptors. And that seems consistent with the fact that you are allowed to fly drones in the more urban parks just not the mountain ones.
As far as I’m aware while it’s illegal to fly a drone from open space (either OSMP or BoCo), it’s legal from Boulder Parks and Rec. So any of the more urban parks are fine. And most (but not all) of the national forest land is ok as well.
Complete no go in Boulder County - https://www.bouldercounty.org/open-space/parks-and-trails/ru...
Complete no go in RMNP - https://www.nps.gov/romo/learn/management/unmanned_aircraft....
Misc - https://drone-laws.com/drone-laws-in-colorado/