Okay, so I have a theory. Why did the Crystal Gems stay with Rose? Obviously Pearl went with her because she's obsessed, but why Amethyst and Garnet? If Rose saw the good in everything, then she would have accepted each of them. So Amethyst was born on Earth, never knew anything about Homeworld, and wants to stay on Earth. What if there are rules preventing Ruby & Sapphire from staying fused for so long? Jasper seemed to dislike fusion. So Garnet didn't want to leave, but had to, to be fused?
“So this is what Homeworld thinks of fusion,” states Garnet in “Keeping It Together.” We don’t know much about Homeworld just yet, but it’s been STRONGLY hinted that fusion is nearly taboo save for times of war and fighting. It’s more than likely this was the reason for Garnet deciding to stay with Rose Quartz because just that--she was loving and accepted every living creature for what it was. If Garnet decided to remain a fusion, she would’ve had Rose’s support.