D.C. Metro has neither the tight curves nor the ridership levels that we have here at MTA NYCT. That is why 75 footers with less doors works for them.
If you have noticed, all end doors on all the 75 footers are locked at all times due to how dangerous it is to cross between them. Even during an emergency, passengers are still unable to use them. Case in point, the shooting last year in which a man opened fire on an r46 N train. People were trying to escape and couldn't since the doors were locked. Luckily no one was killed in that incident.
The seating arrangement on all the 75 footers is also very bad for crowd control. When you're on a 75 footer that is packed to the brim, you don't wanna be standing by those transverse seats as it is very uncomfortable.
Last but not least, MTA likes fleet uniformity. Ordering trains with different doors and seats does not conform to that. These things would require different parts to be used for maintenance, which in turn leads to higher maintenance costs.