>>149529504
Oh, I'm just assuming, I don't think that's the actual official explanation.
I'm pretty sure the writers themselves just don't have any in-universe explanation and simply decided that Rusty shouldn't have a brother, so they decided to not bring Rocky back, which is pretty weird if you ask me, this is exactly the kind of thing that's ripe with potential, not just for Lucy, but also for Rusty, who they have used extensively.
Maybe the show will go on for long enough for Rocky to come back one day.