Why are some substances biodegradable and some non-biodegradable?

Some substances which are degraded and broken down into simpler substances by the microorganisms, are called biodegradable substances. For example: Cotton, wood, paper, wool, etc. Substance (mainly man-made) that do not degraded or broken down into simpler substances by the action of enzymes secreted by microorganisms are called non-biodegradable substances. For example: Plastic, polythene, DDT, etc.

Substances that are broken down by biological processes are said to be biodegradable. Eg: paper and peel of a fruit. But plastic leather etc. are not broken down. These are called Non-biodegradable.