Because it was widely believed that Clinton was a very good candidate and seemed to possess high approval numbers throughout 2013/2014. Also when she got behind the Obama campaign in 2008, it was widely believed by most that in return for an Undivided party, Hillary was to be a shoe in for the 2016 democrat presidential nominee.
She was basically seen as being on par with Biden, meaning that taking on the risk of a 3-way race between 2 centrists and 1 leftist resulting in the leftist winning was unnecessary. If Obama had appointed a Democratic FBI director, and the email investigation was quickly completed, that may have been the case. Instead, he appointed a showman who dragged the investigation out as long as possible and got manipulated by a forged russian document to interfere in the middle of the election.
James Comey is a great man, almost certainly more so than you. Just because someone supports justice equally under the law doesn't make them a showman. Until 2016, Comey was decidedly non-controversial. He didn't want attention - the media forced it on him. He did as few press conferences as possible and kept them all as low key as possible.
