This is a mystery also. There really seems to be no reason for this. Baby Pokemon and legendaries are understandable (as baby Pokemon are just born and legendaries so you can't easly get an army of them) but Nidorina and Nidoqueen have nothing about them to make this happen. It even says in some of the Pokedex entries for Nidoqueen that Nidoqueen are very protective of their young. The only thing I can come up with is that Nidoqueen could be based off of a Queen in real life who was unable to have children. The being very protective of their young could in that case just mean being protective of their country.