Good timing and a growing national weariness over failed drug laws didn't hurt, either.
"Maybe ... the dominoes fell the way they did because they were waiting for somebody to push them in that direction," says Alison Holcomb, the campaign manager for Washington's measure.
Washington and Colorado, both culturally and politically, offered fertile ground for legalization advocates — Washington for its liberal politics, Colorado for its libertarian streak, and both for their Western independence.
Both also have a history with marijuana law reform. More than a decade ago, they were among the first states to approve medical marijuana.
Still, when it came to full legalization, activists hit a wall. Colorado's voters rejected a measure to legalize up to an ounce of marijuana in 2006. In Washington, organizers in 2010 couldn't make the ballot with a measure that would have removed criminal penalties for marijuana.
Since the 1970 founding of the National Organization for the Reform of Marijuana Laws, reform efforts had centered on the unfairness of marijuana laws to the recreational user — hardly a sympathetic character, Holcomb notes.
That began to change as some doctors extolled marijuana's ability to relieve pain, quell nausea and improve the appetites of cancer and AIDS patients. The conversation shifted in the 1990s toward medical marijuana laws. But even in some states with those laws, including Washington, truly sick people continued to be arrested.
Improved data collection that began with the ramping up of the drug war in the 1980s also helped change the debate. Late last decade, with Mexico's crackdown on cartels prompting horrific bloodshed there and headlines here, activists could point to a stunning fact: In 1991, marijuana arrests made up less than one-third of all drug arrests in the U.S. Now, they make up half — about 90 percent for possession of small amounts — yet pot remains easily available.