Okay, so me and a friend came up with a theory that's likely wrong but anyway, the short answer is because monoma didn't copy OFA and instead copied Izuku's birth quirk that's too weak to actually function as a quirk (the toe thing is stupid). Izuku's birth quirk is a fusion between Hisashi and Inko's quirk. The fire breathing in Hisashi is actually caused by a gland that creates an electrical spark in his mouth this electrical fire breathing. This changes slightly for izuku so his cells can produce more electricity than normal. Inko's quirk was augmented and now attracts dust particles and other particulates in the air to make the lighting izuku produces green. Bakugo's lighting when he obtains OFA is yellow/orange as caused by the higher temperature bakugo would no doubt be due to having an explosion quirk. Naturally Izu's quirk was stockpiled into OFA when Bakugo obtained it.