WMATA has hypotheses about why ridership is below projections:

“This ridership drop-off is due to a range of factors, including the reduction in the federal transit benefit as well as reduced overall employment growth and economic growth in the Washington region. Lower gasoline prices may also be having an impact at the margin, especially for riders making longer commute trips and those who park-and-ride (a group which overlaps substantially with the riders most impacted by the federal transit subsidy reduction)… Finally, the impact of telecommuting and alternative work schedules continues, with ridership notably lower on Mondays and Fridays.”

But they might have missed a couple of things (boldface mine):

The document made no mention of consistently unreliable rush-hour service. Over the summer, with fewer eight-car trains available because of additional maintenance needs, rail commuters were treated to a series of meltdowns that turned short rides into headaches. A range of track problems and train breakdowns were to blame, compounded by Metro’s need to single-track around any obstacle.

The transit authority’s own “key performance indicators” from the second quarter of this year told part of the story: Metro missed its targets for on-time performance, fleet reliability and customer satisfaction for both rail and bus systems.

I would also add that the weekend service is horrible. It’s bad enough that during the summer trains on most or all lines run every twenty to twenty-six minutes–unlike Boston, where the summer is the major tourist season, in D.C., tourists typically don’t visit as much as they do in the spring and fall. But post-Labor Day, Metro is still running weekend trains at less than three per hour. Oddly enough, people who live outside the city are tempted to drive in, while residents decide to find an alternative mode of tranportation (or just stay home).