Knifehead (Kaiju)

Some kaiju were very effective in combating the Jaegers. Knifehead's knife went through Gipsy like a heated butter knife through butter. Otachi's acid melted through Cherno no matter how durable its armour was, and her flight was clearly very useful. Leatherback's EMP crippled Striker, the stronger Jaeger, in an instant. Slattern's size and strength speaks for himself.

So knowing how effective these kaiju were, why didn't the Precursors just send more clones of Knifehead, Otachi, Leatherback, or Slattern? We know it's possible because Newt's vision showed several Knifehead copies lined up next to each other. Even complete clones wouldn't be necessary; just their abilities would suffice. Perhaps they could even mix-and-match them; imagine the horror of Otachi with an EMP on top of all her other abilities. Or Slattern with acid and an EMP. Leatherback's EMP is really a gamebreaker here. Put one on even a weak kaiju like Trespasser and that kaiju would be able to take out any non-nuclear Jaeger.

So instead of sending Raiju and Scunner to defend the Breach, the Precursors could have simply sent another Leatherback to take out Striker (again) and let Slattern finish the job.