Ok, so this theory was written last 4th of July originally, so some facts did not apply anymore, nevertheless, it's an interesting thought.
|Date of Publication||10/6/2015|
First off, they could have joined Rose (or rather, Rose took them in) because they had "Flaws" of some degree. Diamonds are regarded as flawless and perfect, right? From what we know of Homeworld, gems that aren't flawless are probably looked down upon or worse. Think about it, Pearl is implied by Jasper to be one of many but is "Defective" and knows how to fight despite her
implied confirmed role as a maintenance/servant class gem. Garnet's component gems would be looked upon as weak by a Diamond (or, seeing how Peridot is small as well, they could be the norm for whatever unknown class of gem they used to be, but we haven't heard that story yet.) and their status as a fusion goes against Homeworld as well. This was confirmed by the recently released Guide to the Crystal Gems. So just in these three, there's a clear pattern of Gems that didn't fit into Homeworld due to imperfections, and the rest of Rose's Army had to have similar reasons for staying on Earth.
It could also have been that apart from adoring humans and the life on Earth, Rose promised her followers a sort of "Island of Misfit Gems" to call home and coexist (or at least tolerate) humans. This obviously failed for unknown reasons.
After Rose "won"
It's now known that The Cluster was the reason why Homeworld left after obliterating the Crystal Gems, but how could they have overlooked Rose's survival? There are a couple of reasons why they ignored her.
- She had no idea The Cluster existed, or if she did, Steven is part of her game-plan to destroy it. She is a master strategist, after all.
- To cover up the rebellion to keep the status-quo on Homeworld. Peridot had no idea the war even happened, or any knowledge of Gems on Earth. Therefore, it's safe to say that it was kept quiet from newly produced gems on Homeworld.
- They know The Cluster will destroy Earth anyway, and didn't want to risk any more able bodied soldiers against her when they could just leave and achieve the same result with better odds.