According to a press release from the National Immigration Law Center:

U.S. District Court Judge Charles R. Breyer found that “the government’s proposal to disseminate no-match letters affecting more than eight million workers will, under the mandated time line, result in the termination of employment to lawfully employed workers. . . .”  The judge also found that “if allowed to proceed, the mailing of no-match letters, accompanied by DHS’s guidance letter, would result in irreparable harm to innocent workers and employers.”

The preliminary injunction is available on NILC's website, along with other information the history of the case.

Read other blog posts about why the rule was a bad idea here, here, and here.