The races and the inter-racial relations in the book's future make zero sense: Why the hell did Moirans and Teklans not intermarry into a single race centuries

The races and the inter-racial relations in the book's future make zero sense: Why the hell did Moirans and Teklans not intermarry into a single race centuries ago? Why are the docile sexless Camites still around? Why are there not one but TWO races that are mainly characterized as being evil (and the characterization is not even subverted in-story)?