Because it was a prototype, just invented, that the lizard mechanic had been trying to get to work when their base was attacked. it wasn't a "hidden reserve,"

Because it was a prototype, just invented, that the lizard mechanic had been trying to get to work when their base was attacked. it wasn't a "hidden reserve," it was a desperate gamble with an untested weapon system. The good guy's effort wasn't a military tactic, it was the suicide attack of defeated individuals spending their lives for any vengeance they could get. Success was unexpected.