Only because Comey said there wasn't clear evidence of her intent to violate the law. Do we really believe she didn't intend to violate the law? She's no dummy.
Director Comey's statement to the press on the FBI’s investigation of Secretary Clinton’s use of a personal e-mail system during her time as Secretary of State.
www.fbi.gov
"From the group of 30,000 e-mails returned to the State Department, 110 e-mails in 52 e-mail chains have been determined by the owning agency to contain classified information at the time they were sent or received.
Eight of those chains contained information that was Top Secret at the time they were sent; 36 chains contained Secret information at the time; and eight contained Confidential information, which is the lowest level of classification. Separate from those, about 2,000 additional e-mails were “up-classified” to make them Confidential; the information in those had not been classified at the time the e-mails were sent."
"Although we did not find clear evidence that Secretary Clinton or her colleagues intended to violate laws governing the handling of classified information, there is evidence that they were extremely careless in their handling of very sensitive, highly classified information."