The culprit is state medical licensing laws. For more, read Cato adjunct scholar Shirley Svorny.
My understanding is that state-level medical licensing laws prohibit doctors from providing charity care outside the state where they are licensed. If so, the Colorado legislature should look into allowing out-of-state physicians, dentists, optometrists, etc., to practice charity care in Colorado. Of course, this is probably more of a problem in geographically smaller states with high population densities near their borders.