The answer has to be related to the fact that those two species of cichlid are, so to speak, young . Lake Tanganyika, the environment where those two species came from, is also from a geological point of view young, being the result of earth s surface movements about 12 millions years ago. Consider that the human species became significantly different from monkeys about two million years ago. However, most mammals started to follow different evolution paths between 65 and 10 millions year ago. Then the lake is young and species populating it are even younger.
Genomes are, still, quite similar in the cells of the two animals.
This results in the chance of interbreeding. The fact that two different species can t interbreed is because of a lot of tiny, slight, insignificant differences hidden in the genomes of two species living close in the wild. There is a point when those tiny, slight, insignificant differences begin to be too much to allow milt of male A species being compatible with eggs of female B species . At this point the two species will be considered different by humans. In reality, the two species started to differentiate from each other long before those tiny, slight, insignificant differences started to add up.