Why would a rough steep mountain habitat have a higher rate of speciation than a large open grasslands habitat?

A rough steep mountain habitat will have a higher rate of speciation than a large open grassland. The reason for that is that the open grassland has monotonous living conditions all through it, thus the species are the same with the same adaptations all over it. The rough steep mountain habitat, on the other hand, has multiple different living conditions. The different living conditions are defined by the elevation, and in accordance to it there will different species living at different elevations. Also, the mountain is a natural barrier for the air masses, so while on one side of it there's rainfall, on the other one there's dry weather conditions, which again creates different living conditions, different adaptations by the living organisms, thus different species.