WHY DIDN"T HE MENTION IT? Because he deliberately didn't want to. I am sure that he was "sweating more than usual" at the time he uttered the words that he did.
In courts, people are to tell the truth, the whole truth and nothing but the truth. Not mentioning his ties with the slimiest nation on planet earth were certain to cause him horrible trouble with that committee on Capitol Hill. He knew it, but didn't want to hurt his own pride. Sessions yearns for the opportunity to be a truly famous man. Well, he is very famous now. If I was appearing before a legislative committee and talking about those things, I'd sure have mentioned my dealings with Russians that he didn't. Despicable country.