The same reason that English Common Laws, such as the Castle Doctrine, are still in American Law. When Arizona was ratified and joined the Union, they didn't scrap everything and start over, the laws of the territory became the laws of the newly formed state. That bedrock of previously established laws were later added to or amended as time passed. Most laws have been updated for the modern times, like in this case, but this prosecutor found an old relic to try and promote his political views despite the legislature updating the law.

Cause their vaccine doesn't work but they won't admit it because that shows that they failed. So instead of just importing working vaccines, they still use harsh quarantines to limit the spread of the disease to a largely unprotected population