If you'll accept second hand info, I've been told that it's a combination of elevation of the bigger population centers, and the proclivity of the populace to enjoying the outdoors at a higher rate than most states
I can't back that up, and it isn't my claim, but I figure it's a decent starting point.
Everyone I know in Colorado is nuts about hiking, hunting, fishing, camping. It's just deeply ingrained into the culture there in a way I don't usually hear about for entire states.
I've heard it said that the weather is a big part of it because, most of the year, it stays well inside the ranges of temperature where you can day hike/ride in regular clothing. No need for cold weather gear. Since there's supposedly also a ton of wild space that's pretty compared to neighboring states, it adds up.