Governments increasingly use randomized controlled trials (RCTs) to test innovations, yet we know little about how they incorporate results into policymaking. We study 30 US cities that ran 73 RCTs with a national nudge unit. Cities adopt a nudge treatment into their communications in 27% of the cases. We find that the strength of the evidence and key city features do not strongly predict adoption; instead, the largest predictor is whether the RCT was implemented using preexisting communication, as opposed to new communication. We identify organizational inertia as a leading explanation: changes to preexisting infrastructure are more naturally folded into subsequent processes.