The recent killing of a marijuana store security guard in Colorado and the wounding of another guard in San Bernardino are the latest examples of the crime lure posed by cash-only pot dispensaries, industry observers say.

But while those armed robbery attempts and a pot store shootout in Walnut Park in Los Angeles County were unfolding last month, Democratic-sponsored legislation that could have led to more dispensaries offering plastic and electronic payments in lieu of cash was blocked by House Republicans.

“Moments such as this,” said security expert Michael Jerome of Blue Line Protection Group, referring to the recent killing of 24-year-old Colorado pot store guard Travis Mason, “remind us that the cash-based nature of the legal cannabis industry here in Colorado makes these dispensaries and cultivation facilities prime targets.”

A former Marine with a wife and three children, Mason was shot in the head on June 19 by two armed robbers at the Green Heart pot dispensary in Aurora, a Denver suburb. It was his second week on the job.

The would-be robbers, who fled without any...