If residential fire sprinklers work, why have most states dropped the requirement?

Adoption is a question of politics, not evidence. The International Code Council has kept the provision in every revision since 2009, and the model code is written by the same body whose codes those jurisdictions otherwise adopt wholesale. If sprinklers weren’t effective, the code committee would have removed it. California and Maryland kept the statewide requirement. 20 other states, Colorado among them, left the decision to local jurisdictions, and the Colorado jurisdictions that have kept it are the ones on the Front Range. Codes that amend out residential fire sprinklers are, by definition, substandard. That’s not the Colorado way.