When Baily is fit, he immediately goes into the first team.
When Smalling is fit, he sits on the bench. That's the difference.
And regarding Smalling last season: under LVG's ultra defensive style with 2 defensive midfielders, he was good. No doubt about that. But the moment he was asked to defend with in an attacking team, he fell to pieces. Remember that LCFC game, back when our team was built to attack (while he was on the pitch, we conceded 4 goals)? And recently, under Jose, he was part of the team we conceded 4 goals.
I think in an ultra defensive setup, Smalling has proven to do well. But when we set up to attack (where our CBs may be left isolated and will probably be required to go 1v1 with an attacker), he is extremely poor.
Both P.Jones (when fit) and Rojo, have proved that they can handle opposition attackers, while our midfielders focus on attack.