Why are most fertilizers rich in nitrogen and phosphorous?

Most fertilizers are made from animal (Cow most of the time) waste. The animal waste is high in animal byproducts which are mostly nitrogen and phosphorous.

I apologize that I can't give more of a specific answer without getting more background information for your unit.

Cause nitrogen causes plants to grow very well. I don't think plants can do anything with nitrogen gas, the only way they can get it is through the roots, called fixed nitrogen. In nature the only things that fix nitrogen are lightning and microbes that exist in roots of certain plants. So nitrogen is rare in nature and is a limiting factor in plant growth.