I guess Tuchel just feels like they're overstocked with centre halves. Somehow Stones, who never plays, is ahead of Branthwaite, as is Quansah who is now at 6th in the Bundesliga Leverkusen. Guehi is an automatic pick, and Chalobah has been in the squad for quite a while now too. Konza is another automatic pick (although I don't rate him, looking at it objectively he's been a mainstay in Villa's defence and they're having a great season), so that's 5 centre halves already. Dan Burn (lol) has been in the squad a while too, which makes 6.
Then Branthwaite has been out for 6 months, so would need to be proving himself over a period before coming back into contention.
IMO he should definitely be in there over Quansah and Burn at a bare minimum. And he's clearly a better defender now than Stones is, who is past his best and backup at City. Better than Chalobah too. And Konza.
Haha.