The most plausible hypothesis to explain why species richness is higher in tropical than in temperate regions is that

The most plausible hypothesis is: Tropical regions generally have more available water and higher levels of solar radiation. They also have very high rates of immigration and very low rates of extinction.

explanation:
Water is essential for an abundance of life, and sunlight is the means by which the whole food chain receives energy. Plants convert the sunlight into food, herbivores eat them, and other animals eat herbivores.

Hope this helps, and muchas luck on your biology.