Shivans are stupid. Why didn't they send a Sathy to blow up earth? No one could fight the Sathy at that time. It think it would take the entire GTVA just to blow this up. Then the Shivans would sned another one. Same story. Gradually, there would be no fleet to defend the home systems, so shivans just go in and boom. If we had 80+ juggernauts, we would do the same to them. Silly Shivans.
It's obvious: they weren't available.
Either that, or perhaps... well, perhaps a Sathanas is far too valuable to waste on an invasion. Perhaps the first Sathanas we destroyed was simply the first to arrive. It arrived first, taking advantage of its massive firepower to swat away the Tatenen, which had been threatening defenceless (hah!) Shivan gas miners. When GTVA warships showed up and attempted to destroy it, the Sathanas again capitalised on its advantage and destroyed them, making its way to Capella. When confronted by the Colossus, it had no choice but to fight, perhaps thinking that the Colossus had similar abilities. However, it was destroyed, thanks to Alpha 1. The rest of the Sathanas fleet arrived shortly after one another, not realising how close the Colossus was to defeat or that it was the only one the GTVA had.
Perhaps, the Shivan supernova device was experimental. The Sathanas fleet had never been required to destroy a star before, and the energy caused some of the juggernauts to malfunction, resulting in the death of many Shivans aboard the juggernauts. The other ships, perhaps, had never intended to survive the supernova, only buying time for their comrades.
That, or the supernova was an unintended side-effect. Maybe the intent was to focus the star's energy in such a way as to create a huge subspace node, but never to destroy it. Some of the Shivans realised what was going on and managed to warp in time, but others were either drained by the device's energy requirements, or their warships were too far from the newly-created node to jump.
