The complexity and variety of organic molecules is due to

Due to the chemical versatility of carbon atoms

Explanation:

Carbon is a very important element and it is commonly found in many varieties of chemical compounds. They are also found to be present in space in various amount. It is often considered to be versatile because they are capable of forming single, double, as well as triple bonds. This enables them to make single and branched chains. They also form ring chains when combined with other atoms of carbon.

It is because of this versatility of carbon atoms, there forms complex variety of organic molecules.