No cause for the supposed delay has been given. It's equally possible that March was always an enthusiastic prediction and never likely to happen.
During an interview with I Am Rogue that ran earlier this week, The Avengers and Captain America: The Winter Soldier co-star Cobie Smulders was asked when she might be joining the production. Her response, "I don’t know the exact dates. I know it's some time in March or June," now seems a bit less personal and more likely applicable to the shooting schedule in general.
CBM further notes that, per a podcast interview with the movie's star Chris Evans, he doesn't yet have his script for the film, but has enough of a knowledge of the story to confess there will be some other Marvel cameos. In addition to Maria Hill, played by Smulders, Samuel L. Jackson has been confirmed to return as Nick Fury. Sebastian Stan will come back as Bucky Barnes, as well, and Anthony Mackie will join the cast as Sam Wilson, the Falcon.
Captain America: The Winter Soldier is due in theaters April 4, 2014.