I know my friend from Utah when went to buy in Colorado had to use cash to buy there. They did not wanted her to use credit cards.
That could be for other reasons. Colorado (like other states who legalized a few years back) has a big problem with over-supply, which drives prices and profitability down. I've heard of dispensaries who combat this by encouraging patrons to pay cash, then they under-report the cash transactions to avoid steep taxes.