Why is there such great diversity among organic compounds

It's because all organic compounds have both hydrogen and carbon. Carbon can make up to four covalent bonds with its four valence electrons; this lets carbon bond in thousands of ways with thousands of other elements, making organic compounds very diverse indeed!

Carbon has four bonds right? So if a carbon attaches to hydrogen atoms,An organic compound is formed. However if a carbon attaches to another carbon and that carbon attaches to other carbons, there will be a lot of new compounds. So that's why there is quite a diversity among organic compounds. Hope this helps!