The major-league markets that have two teams -- New York, Los Angeles, Chicago and the Bay Area -- don't like it much when both teams are in town at the same time. Not that there's that much overlap in the fanbases, but it's easier for local news coverage, and makes for better scheduling in general -- for example, if it rains in one of those cities, you'd rather have just one delayed or postponed game, instead of two.
That's why it's unusual to have a circumstance such as the one now -- when both New York teams are scheduled at home for a three-game series in September. Usually, such overlaps come earlier in the year. And later in the year is when weather often interrupts the schedule; with fewer chances to make up games, having the Mets and Yankees both at home this week risked a double postponement, and that's exactly what's happened Tuesday:
The strong winds and severe weather threat has caused the Yankees and Mets to cancel tonight's games— NY1 Weather (@NY1weather) September 18, 2012
UPDATE: That Nationals/Dodgers game has been postponed, and rescheduled as the opener of a straight (i.e. one admission price) doubleheader, beginning at 4:05 p.m. Wednesday afternoon.