why are plants different ?

Your question seems to be incomplete. 

What I interpret from your query is that you want to ask why all plants are not same. Plants living in different areas are different from each other. Plants growing in hot weather like deserts are different from plants growing under water. 

For example: Cactus plants (found in deserts) have the following features which are not present in  plants growing in other areas.

1. Roots grow deep to get water from the ground.

2. Leaves reduced into spines so they don't lose water.

3. Stems are thick,spongy and green to store water and prepare food.