You Don't Normally Build Giant Monsters To Use Once

Back to Steven Universe for a minute. At some point during the original war between the Crystal Gems and Homeworld, the Diamonds began a project for a massive weapon called the Cluster. They forcibly fused millions of shards of shattered gems into one immense being, shards with just enough awareness to remember they were someone once, but unable to recall or regain that, and left it to gestate deep beneath the Earth's crust. The idea being, when it was ready, the gem shards would create a huge fusion creature that would tear the Earth apart as it emerged.

Before things ever got that far, the Diamonds decided to cut their losses and basically nuke the planet from orbit. They weren't trying to destroy the world, just every gem still on the planet after they ordered their forces to withdraw. It didn't work - the gems in question wound up corrupted with greatly altered forms and little sign they remember who they were - but as far as the Diamonds seemed to know, they'd killed all the rebels and that was that. By that point, the Cluster was basically a final "Fuck you," to a planet that had given them a lot of trouble, and gotten one of their one killed.

That seems like a lot of trouble to go to for just that. Once the Earth is destroyed, they're still left with the planet-destroying fusion monster roaming the cosmos. Maybe they didn't think that far ahead, or they don't have any worlds anywhere near Earth, so the Cluster would never pose a threat to their interests. Still, they sent Peridot to Earth to check on its progress. Maybe that was impatience, although the Gems' lifespans mean they consider centuries to be hardly any time at all. That they'd bother to monitor it suggests they had an interest in the success of the project beyond simply destroying Earth. So what is the end goal?

There's no evidence currently Homeworld has experienced further rebellions since that darn Rose Quartz. The Cluster isn't a pacification weapon, anyway, unless you operate on the same wavelength as Emperor Palpatine, where the sheer scale of the weapon wipes away any hope of resistance just by showing up. That would require the Diamonds to control the Cluster, and be able to transport it through space. Having succeeded once, they could create more, but if it's going to destroy the world when it emerges, again, not a great pacification technique. It kind of ruins the world as a source of resources, which Peridot said Homeworld is running low on, anyway. How far are the Diamonds willing to go for spite?

Up to this point, the only two intelligent species we've seen in the show are the Gems, and humans (assuming you count us as intelligent). We've seen the remains of another colony, when Stevonnie was stranded on it, but whatever civilization we saw seemed to be Gem-related. This doesn't mean there wasn't a preexisting civilization there, or that there isn't one on another world they colonized. Or maybe a world they tried to colonize, and were rebuffed. Because it was already being run by another species, and the Diamonds want something to cope with them. Just because Gems and humans are the only species we've seen so far, doesn't mean they're the only ones out there.

(In this, my mind keeps going to the Next Generation episode where Q decides to make a point by throwing the Enterprise in the path of the Borg. Although it could really be any number of Star Trek episodes. They are constantly running into terrifying new shit in that universe.)

The major flaw in this is we've never heard a Homeworld gem make reference to any sort of conflict, other than the rebellion for Earth. But we also know the Diamonds tend to withhold or distort a lot of information. The various Fusions Steven and Lars encountered on Homeworld all thought Earth had been destroyed thousands of years ago. Pink Diamond and Pearl had no idea two different types of Gems could fuse until they saw Ruby and Sapphire form Garnet. Peridot (and I'd imagine most other Homeworld gems) had no idea there were "unauthorized"fusions on Homeworld at all.

It's possible that, other than the Diamonds, the only gems that would know there was any sort of a conflict, would be the ones fighting in it. If the Diamonds ordered any survivors to keep their mouths shut about it, they most likely would. Although I doubt Gem soldiers get leave. They probably fight until they're not able to, and then they're cast aside. Or their pieces are combined into a new Cluster.