He was an associate of Trump during the campaign.
Meeting with Russians isn't bad, but lying about it under oath is, which is what Sessions did.
Did you watch any of the Sessions confirmation hearings? I did. It was one of the more interesting ones.
The entire thrust of the questioning was about his role and relationship to Trump, and his prior work as US Attorney with complaints about his civil rights record. His role and position as a Senator was not questioned - it was only mentioned by some of the Republicans on the panel as part of their lead-in remarks, praising his tenure in the Senate. It therefore seems quite plausible to me that any question about meeting with Russians that he was asked was being asked in the context of his role and relationship to the Trump campaign, not in his role as a Senator and a member of a key committee that deals with foreign governments.
That he, as a Senator, along with multiple other Senators (from both parties) met with the Russian ambassador (and other foreign government representatives) during the yearlong campaign cycle is unrelated to his role of being a Trump supporter. If he had resigned from the Senate and taken a paid position in the Trump campaign that would be a totally different matter. But that was not the case, and he had every right (and responsibility) to perform his senatorial duties, and based on his committee role that meant meeting with representatives of foreign governments.
I think it is good that he recused himself from the justice department's investigations into the Russian meddling issue, but he did that to remove any hint of impropriety that his detractors would raise, vailid or not.