The younger people (< 45) in CO are largely transplants. Many moved because of the outdoor life, pot legalization and because they visit friends and enjoy the culture. I think that younger influence plays a big role offsetting the traditionally conservative farmland.

Pedantically, though, the votes haven't been strictly progressive, more lowercase-L libertarian. Often the state votes to increase freedoms instead of traditional progressive/conservative lines.

Overall, i think the CO population is more drug-friendly than the average liberal US voting pool. Denver feels relatively moderate, politically, as we have so many influences from where people came from (east coast, california, texas). We have one of the most liberal (Boulder) and conservative (CO Springs) cities in the US. The things that the population collectively agrees upon is often the freedom-y bits.