Like others have said, Red Rocks is a big part of it, but Colorado is also a bit of an island of civilization in the western part of the country. We're surrounded by Wyoming, Nebraska, Kansas, Oklahoma, New Mexico, and Utah - which probably aren't the states you think of first when you think of budding metropolises. So lots of events are held in Colorado that attract people from neighboring states.

Also, Denver International Airport is one of the busiest airports in the world since we're a huge connection hub to the rest of the country. So it's very easy to travel here.