Because it was against the laws white people made up. Blacks could not apply for patents. They couldnt vote to change the laws. White people that were too dumb to make their own inventions stole their ideas and were able to patent them and get rich. Whites need that help because they were not smart. Face it. You white guys are not innovators. You steal everything and claim it was your idea.
"Holt ruled in 1857 that slave inventions couldn’t be patented under existing law. As a slave, Montgomery wasn’t a citizen and was therefore “legally incompetent,” in Holt’s words, to file a claim on his own. At the same time, because neither Jefferson nor Joseph Davis was the “true and original inventor” of Montgomery’s propeller, neither could file a claim on the slave’s behalf and thus legally protect the invention."