Obama always cared about appearances. Investigations at that level also take a long time. You have to remember that they thought Hillary had it in the bag. Giving the appearance that they were using prosecution to stifle a political opponent would have been dangerous. Media rumormongering and selective leaks are par for the course for both parties, but directly prosecuting opponents is still a bridge seldom crossed.

Plus, always remember that Comey was on Trump's side until Trump got elected and started demanding oaths of loyalty and interfered in investigations.

For who could be free when every other man's humour might domineer over him? - John Locke (2nd Treatise, sect 57)