Comey's whole deal was transparency. He stepped over all kinds of lines, in what he said was due to major public interest in an effort to be clear with the public. In that press conference, he criticized Clinton, he took the case completely out of the hands of the Justice Department, he gave a detailed description of how he came to his conclusion, etc. So the door is already open. If he knows something -- ESPECIALLY since he informed Congress that the investigation was completed -- and it turns out that it's significant, and it leaks....well, there goes that transparency. He would be accused of playing favorites: being transparent for one side, but staying mum for another. Which is why I see why he sent the letter. "We found more emails and they might be related."
It would have been great for him to give a more detailed account of what they found in the laptop (that it's about Abedin and that it's regarding the Weiner investigation), but I'm just assuming that he can't. He was just as vague in his memo to the FBI employees about the investigation, so that suggests to me that he can't be as detailed as everyone wants him to be.