The airport charts are updated on a 28 day cycle. Say the PAPI system is completed on day three of the cycle. The airport authority or FAA wants to turn it on. (Imagine what would happen if a plane landed short during the 25 days till the next chart update and people found out there was a system installed that could have helped but wasn't turned on.) But for the next 25 days crews briefing off the chart are not expecting to see a PAPI. The NOTAM is issued to update the crews that something has been added.