Sessions is a former senator who is used to somber debate. He is too "staid" from his days as an unadventurous talker. He is not a risk-taker.
Trump, on the other hand, is an aggressive, risk-taking businessman.
Trump needs an aggressive law-enforment type in the AG spot. Sessions doesn't appear to be the right guy to me.
I know many people here don't like him, me included, but Chris Christie fits that description.