All those Californians are spoiled and soft due to the weather. Once the thermometer dips into the 60's their jackets and gloves won't keep them warm enough.
Just kidding. I grew up in Simi Valley. I absolutely love Denver as I enjoy mountain sports too, but I like cycling so much that every winter I feel a slight pang of regret thinking that perhaps I should try to make it back to Cali. Not the least of which reason is actually for In-N-Out :-)
But then I visit and see the traffic and cost of living and I look out my window at the snow capped Rocky Mountains and feel like I found my new home. Back to the OT, there are definitely less riders now than in summer but I still see a fair amount of cyclists. It depends on the route though, they mostly stick now to the well-traveled areas.