This is not my puzzle, and I don't remember the source, but I don't see it posted here yet, so...
Update: After reading some of the alternative answers, I realize this puzzle would be better with an elevated station, not a subway, so I have included both versions. Either version supports the accepted answer:
Original puzzle: 1) A subway station has three escalators running side-by-side. One escalator only descends from the street to the subway platform, and the other two escalators only ascend from the subway platform to the street.
Better puzzle: 2) An elevated train station has three escalators running side-by-side. One escalator only ascends from the street to the elevated platform, and the other two escalators only descend from the elevated platform to the street.
In either case: Why aren't any of the escalators rigged to switch directions, based on time of day, train schedules, etc?