Why are there many niches of species in one mountain

Due to different climates as a result of elevation change.

Explanation:

Due to the change of altitude in the mountain, it grants huge diversity of climates, which in return favor the diversification of species that are inhabitant to that environment. The climates and the local variations that has been determined by geomorphological, edaphic and vegetal cover features gives rise to variety of microclimates which later becomes habitats colonized by species which then adapt to that climate and also adjusts that specific niches.