Ken Rosenthal asked a great question, everyone is appalled one voter didn't vote for Jeter but Jeter was a lock to make the Hall so what if the one voter strategically decided that he really believed there were 11 names who deserved a vote and removed Jeter so that he could vote for all 10 who truly needed the votes more than Jeter did?
Wouldn't that be an acceptable reason to not vote for Jeter?
Maybe it depends how much you value being elected unanimously which is kinda a superficial honor anyways. It's not like this one voter prevented Jeter from getting elected.
Maybe they would be ok with revealing themselves if they weren't the only one. I have to believe they're confident in their vote but there's no way they knew they would be the only one so at this point it's probably best to back down and just avoid what would be coming to them. So in that sense it's not cowardice as much as damage control.