The reasoning behind this is quite simple. Most regions and orgs have a 3 letter acronym, but because of that we cannot search for mentions of them on RMB posts as the search is limited to 4 letters minimum.
Granted, there's probably a technical limitation involved that makes searching for short words very expensive, however I believe that it would be very useful to be able to search for acronyms, as it helps grasp the political landscape more easily by knowing which region said what about another region or organization.