We’ve seen enough of the Gem homeworld to be able to make some assumptions about it. As we get more information our guesses should become more accurate, but these aren’t wild stabs in the dark: it all meshes with what we’ve seen previously to paint a consistent picture of the modern homeworld.
In short, the homeworld is militarized but weak.
A (currently) weak homeworld
Thousands of years before the events of the show, the Crystal Gems waged what appears to have been a large-scale rebellion against the homeworld. The armies on both sides were very big, and in the end the homeworld was only able to eliminate the Crystal Gems from a distance.
Despite having, as far as they knew, wiped out all resistance, the homeworld was distracted from Earth for more than 5,000 years. Even when alerted to the fact that at least some Crystal Gems were still alive, they sent only the tiniest fraction of the force that had previously fought for control of Earth: a single ship crewed by a single technician, carrying a single soldier, and a bringing single prisoner along as a source of military intelligence.
It is true that Peridot had only reported there as being three Gems in the Kindergarten control room, but there could easily have been more elsewhere. Even if the homeworld had correctly divined that there could not likely be many Crystal Gems – or else they would have made a more obvious presence on Earth – or been told by Lapis Lazuli that there were only three Crystal Gems, sending just Jasper was a display of great confidence: while it’s true that she was able to win the initial fight using the surprise of the Gem destabilizer, we also saw that Garnet was able to beat her alone—and Peridot certainly wasn’t a match for Amethyst and Pearl once disarmed.
Of course Jasper implied that she had only gone because she was looking for Rose Quartz, or at least a larger remnant of her army; she also came in a ship with the capacity to take dozens, if not hundreds, of Gem prisoners. When she saw that the there was no larger army waiting to fight, she told Peridot to simply use the ship’s main weapon to destroy the Crystal Gems.
This leads to a bizarre conclusion: Jasper’s only purpose was to fight the Crystal Gems if they were too strong or numerous to be killed from the ship, but it would probably take only a few Crystal Gems to defeat her even without fusion (a technique she certainly remembered them using during the war).
Why send a soldier if the only situation she would be needed in was one where she wouldn’t be strong enough to complete the mission? Why send a single ship? There could not have been a hope of taking the Crystal Gems by surprise: they knew the Crystal Gems were alerted to the imminent attack, the ship made an obvious and powerful sonic boom upon entering the atmosphere, and the ship approached the Crystal Gem base directly.
We’ve hypothesized before that the homeworld had been involved in a large-scale war for thousands of years and was only recently free to reclaim lost holdings. If so, they probably lack the military resources to devote to Earth. That would certainly explain their tepid response. The Gems control planets throughout the universe: overpowering three Gems on a backwater planet should have been a simple matter for them if they weren’t overstretched, yet they sent only the bare minimum to probably complete the task rather than enough to ensure the enemy was crushed.
Steven, a complete outside-context threat for the homeworld, may have been vital to the Crystal Gems’ escape, but it’s not hard to imagine their first fight with Jasper going differently: had Jasper been disarmed rather than landing a hit on Garnet, who was not expecting to face such a deadly weapon, Steven’s escape from his prison cell never would have been necessary. Of course, had there been ten Jaspers instead of one, neither his escape nor a Crystal Gem victory would even have been even remotely possible.
A great change in personality
The Crystal Gems and the modern homeworld are from completely different eras. The Crystal Gems seem to come from a Gem culture modeled on literary ideas of lost, ancient, highly advanced societies by the show’s writers. These societies are usually based, visually, on ancient Greece or Rome. They typically enjoy great surpluses (which the Gems would have by default since they need no resources or labor to sustain their existence) and so devote large amounts of effort to the arts and sciences; their culture flourishes, and the environment is pristine. Such societies are usually portrayed as utopias.
The modern Gem homeworld, from what little we’ve seen of it, has a different literary basis: one very dystopian in nature. The model appears to be taken from sci-fi tropes about aggressive, technologically advanced societies. Rather than focus on culture, these societies focus on military matters. Pollution is probably rampant as the focus is on pure economic production (to some planned, normally military, end, rather than private consumption). All in all, not a great place to be.
Five thousand years is a long time, but Gems are a long-lived species as well: one would expect there to be more continuity to their culture. It’s possible that it changed over time naturally, but even if we didn’t know that they had lost many planets it still wouldn’t be hard to assume that there was some kind of pressure forcing such an enormous re-ordering of their society in a direction that is less pleasant to live in though more effective at directing effort towards threats.