Why are Nidorina and Nidoqueen sterile, while female Nidoran can lay eggs?

i don’t f$%^ing know; I’m sure someone’s asked me this before but I can’t be bothered finding the link to the old question because I have had too much gin.  I think I made noises about the grandmother hypothesis in human longevity which will answer all of your questions if you google it. (hint: it won’t actually answer all your questions; I was lying; it will actually raise more and better questions)