No certain reason. The FAA assigns codes and often they try to make it recognizable to the city, but often it's really not for example Sacramento CA has the code SMF which if you didn't know it would not immediately tell you the city. Chicago o' Hare is ORD. There are reasons however because the F in SMF is for Field and ORD is short for Orchard, which is the property they built Chicago airport on. The property was previously Orchard Place where originally they built aircrafts.
The three-letter airport codes are created by using a combination of letters representing the airfield, city or region’s name. Small, general aviation airports may have a combination of letters as well as numbers. RDU falls right in line with the pattern for codes like SFO for San Francisco International Airport
It doesn't stand for anything. In the 1960's the FAA made this assignment. Its similar to the O in SFO
Raleigh-DUrham International Airport
It's for DUrham, which is part of the metro area the airport services.
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