. . .because it published a Romney op-ed on its front page:

The White House Press Office has decided to keep the Boston Herald from reporting on today’s Boston fund raiser for Barack Obama’s re-election campaign efforts.
Evidently, the White House felt that the paper has demonstrated a bias in its history of reporting on the President. In response to the Herald’s press request, White House spokesman Matt Lehrich pointed specifically to the paper’s decision to run an op-ed piece by form Massachusetts governor and potential 2012 Republican candidate Mitt Romney as a front-page story.

Full story at Mediaite.