Development of the zygote proceeds abnormally and the hybrid is aborted. The hybrid is healthy but sterile. An abbreviated illustration of meiosis, by which reproductive cells duplicate to form gametes. So species remain distinct due to reproductive isolation. This question is critical, because it is what produces many species from few, and results in evolutionary trees of relatedness.
Geographic isolation leads to reproductive isolation.
Once two populations are reproductively isolated, they are free to follow different evolutionary paths. Temperature, rainfall, predators and competitors are likely to differ between two areas 100's or 1,000's of kilometers apart. Thus, over time, the two populations will differentiate. Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population.