Nepenthes divided out from a group of plants which have perfect flowers, dividing into male and female was very likely one of the mutations that lead directly to the evolution our current Nepenthes. All other related genera have perfect flowers so it makes sense to assume this is a rather recent development. It is thought this happened when two different kinds of barely compatible species hybridized, but incompletely, dividing the resulting species into either male or female individuals while doubling the number of chromosomes. I believe this allowed Nepenthes to evolve faster as now fewer individuals can hold more genetic diversity as compared to individuals of Drosera, Dionaea, Aldrovanda.