I think it has something to do with how NMSC calculates cutoff scores, and the small number of people in DC. Below is NMSC's description of how it sets cutoff scores (with key parts underlined). I'm guessing that if NMSC followed this same method for calculating a cutoff score for DC or the other special "selection units," it might result in those selection units getting an unfairly small number of semifinalist slots (even fewer than they get now using the "highest state score" cutoff). For example, tiny little Rhode Island only had 8,200 seniors take the SAT last year, and DC had only half that number (4,100).
Maybe I'm wrong about this, but that's my best guess why there's a different standard.
NMSC designates Semifinalists on a state representational
basis to ensure that academically able young
people from all parts of the United States are included
in this talent pool. Using the latest data available, an
allocation of Semifinalists is determined for each state,
based on the state’s percentage of the national total of
high school graduating seniors. For example, the number
of Semifinalists in a state that enrolls approximately two
percent of the nation’s graduating seniors would be about
320 (2 percent of the 16,000 Semifinalists).
NMSC then arranges the Selection Index scores of all
National Merit Program participants within a state in
descending order. The score at which a state’s allocation
is most closely filled becomes the Semifinalist qualifying
score. Entrants with a Selection Index at or above
the qualifying score are named Semifinalists. As a result
of this process, Semifinalist qualifying scores vary from
state to state and from year to year, but the scores of all
Semifinalists are extremely high.
In addition to Semifinalists designated in each of the
50 states and without affecting the allocation to any
state, Semifinalists are named in several other selection
units that NMSC establishes for the competition. These
units are for students attending schools in the District of
Columbia, schools in U.S. commonwealths and territories,
schools in other countries that enroll U.S. citizens,
and U.S. boarding schools that enroll a sizable proportion
of their students from outside the state in which the
school is located. A participant can be considered for
Semifinalist standing in only one state or selection unit,
based on the high school in which the student is regularly
enrolled when taking the PSAT/NMSQT.