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Technical game reasons.
When making GSC, Game Freak connected Nidoking, Nidorino, and Nidoran M to Nidoran F, but could'nt also connect Nidorina and Nidoqueen to Nidoran M, or else Nidoran M would've been able to breed all three Nidoran F stages when bred with Ditto (and viceversa), due to being connected with all three.