Bismuth was kept bubbled for so long because she’s an extremist.

She wanted not only to defend the Earth and the Rebellion, but to hurt and shatter her enemies. She wanted to destroy every single Homeworld gem, even fleeing foes.

And even neutral bystanders like Lapis Lazuli

That’s why Rose had to keep her locked up in that bubble. She couldn’t let her roam free and exact her extremist ideals.

In this season, Steven will eventually discover Bismuth’s extremism when she comes in contact with Peridot and Lapis and tries to kill them. And he and the rest of the Crystal Gems will have to bubble her again.

But Steven is nothing but the master of redeeming other characters, so I think he’ll eventually get through to her.