Mostly because of the Front Range of the Rocky Mountains.
Circa 1988, I flew a deHavilland Dash 7 (“Quad Otter”—see below!) for Rocky Mountain Airways. All that weather and those winds like to spill over the Front Range and into Denver, swirl around and create all kinds of mayhem lol. Once, a jolt of severe turbulence actually shut down one of our engines! (We had 3 more, and barely noticed! And, we were able to get ’er restarted right away!)
But, most times, the turbulence around Denver is benign, if dramatic.
Buckle up!