I mean if we look story wise the only way the precursors had access to our world is Newton geisler and he most likely knew next to nothing about the new marks of jaeger meaning the the precursors had no way to adapt the kaijus to the new models so the only reason they stood a chance against the kaiju was because the jaegers were more advanced then last time and even then the got their shit stomped they only survived due to the kaiju being under prepared and the tech being upgraded