Why are the classification systems changing every now and then?

(N/A) Millions of plants,animals,and microorganisms are found on Earth.
Many of these have been identified by scientists,while many new species are still being discovered around the world.
Therefore,to classify these newly discovered species,new systems of classification have to be devised every now and then.
This creates the requirement to change or update the existing systems of classification to incorporate new biological data.