"If it wasn't for your mother's shield, kiddo, I dunno… In the end she was only able to save a few of her closest friends…" – Greg, The Return

Rose's shield was able to protect against the Superweapon, but only Garnet and Pearl were with her at the time. Amethyst was safe because she was still in the ground, and Lapis was saved because she was in the mirror.

And Homeworld didn't come back because they had already planted the Cluster in the Earth's core…

The scary thing about this is that this proves that the Diamonds have the power to turn Gems into Monsters… If the Diamonds come back to Earth they could turn any of Steven's Mommas into Monsters.