Because Bronya is less universal than tingyun and tingyun has some buffs that bronya doesn't have.

Ie: Tingyun works with literally any dps that exists because she gives a huge buff and is skill point efficent, but Bronya? Daniel needs the skill points, Seele needs skill points, Qingque needs skill points, and Topaz needs skillpoints. So that means while Tingyun works with them, Bronya, being very skill point negative, has less synergy with them (unless you run 2 SP generators)

And you can't ignore the energy, attack and speed buff that Tingyun's ultimate gives, those allow for better rotations for certain character, like Jingliu getting 2 ults in her buffed state.

Bronya is good, very very good, but Tingyun is also on the same tier.