What are the reasons for tremendous biodiversity in tropical rainforest?

Warmer climate allows for things to grow throughout the year, and nearly 90% of land animals are able to adapt in hot weather.

To elaborate on your answer, energy must enter an ecosystem by being converted from sunlight. Plants convert sunlight, water, and carbon dioxide into glucose in a process called photosynthesis. Since the sunlight is greater in intensity in the tropical (equatorial) areas than in the cooler northern, there is more energy available for photosynthsis for various kinds of plants. Since the plants are now producing more energy, there can be more sustained animal life than in the northern areas.

Easily adaptive weather, better soils, better plant life, better food diversity. Same reason why people choose to live around beaches than in the arctic.

This is because most plants and animals like and thrive in warmth and need moisture to survive. Some can survive in both warm and cool environments, fewer can survive in cold environments so the reason is that that area is conducive to life more than other areas. That is not to say that there aren't plants and animals that don't like hot environments (boreal forests for example are not found in the rain forest and polar bears don't care too much for miami beach either).

Two other reasons in case they haven't been mentioned yet.

Climate: Tropical rainforests receive almost 12 hours of sunlight every day. This sunlight is converted to energy by plants through the process of photosynthesis. Since there is a lot of sunlight, there is a lot of energy locked up in the rainforest. This energy is stored in plants that are eaten by animals. Because there is a lot of food, there are many species of plants and animals.

Canopy: The spreading structure of the rainforest canopy means there are more places for plants to grow and animals to live. The canopy offers many unique sources of food and shelter. For example, there are plants in the canopy called bromeliads that store water in their leaves. Frogs use these pockets of water like ponds for hunting and laying their eggs. Since all their needs are met in the canopy, some frog species spend all their lives there.

Thanks.