Why are some substances biodegradable and some non-biodegradable ?

The substances which can be broken down in simpler substances by some microorganisms like bacteria or decomposers are called biodegradable substances. For example: dead remains of plants and animals and their waste products, paper etc.

But some compounds like plastics and metals can’t be broken down or decomposed by microorganisms, these compounds are known as non-biodegradable compounds.

So, it depends on the microorganisms(decomposers) of being specific in their action that some waste materials are biodegradable, whereas others are non-biodegradable.