Session recused himself because it was the right thing to do. Even though he knew it was not a legitimate investigation all along, his participation in it would represent a conflict of interest.
“I should not be involved in investigating a campaign I had a role in…"
When the FISA documents reveal the actual witch hunt and everything else, there will no longer be an investigation that (involves the Trump campaign) and then there will no longer be a reason for his recusal.
He followed the rules. Now comes the pain.