Why are some substances biodegradable and some non-biodegradable?

Some materials are biodegradable because they can be decomposed by micro-organisms/decomposers. For example-substances like paper, vegetable peels can be decomposed by micro-organisms into organic compounds, hence they are biodegradable.

On the other hand, some substances are non-biodegradable because the micro-organisms/decomposers cannot break or decompose them. For example – polythene bag cannot be decomposed by micro-organisms/ decomposers and hence they are non-biodegradable.