Actually, if you read the transcripts of his Senate confirmation hearing, Sessions recused himself from anything having to do with the 2016 election, not just the Trump campaign.
One and all have interpreted that to mean that Sessions is ethically incapable of conducting a fair and unbiased investigation of the president's rival, Hillary Clinton.
I hear what you're saying, though. Why can't he investigate any of the Clinton's other myriad crimes, like ServerGate and Uranium One? She could be serving a life sentence for the former, and hung by the neck for the latter.
Eh, personally I think Uranium One is a goose chase, but the server and the foundation are ridiculous! If he did recuse himself from investigating the Foundation, then we really need to find someone at doj who will get in there!