I would have to go over a detailed transcript, but did Sessions not say earlier that Comey was fired because he did not vigorously prosecute Hillary Clinton based on evidence before him? This seems self-evident but I don’t recall hearing that opinion placed on the public record before today.
For why Comey fired, refer to Rosentien memo. That is what Sessions has been doing. That memo does NOT say “lack of prosecution.” It says usurping prosecutor function, testified he would usurp prosecutor function again, and breached privacy rules by going public with the results of an investigation.
Because it was not up to the FBI director to prosecute.