Because its feature and some other properties did not come in the catagory of microorganisms . virus are dependent on other organisms to reproduce it only reproduce when it enters in a body of living bieng then only due to these reasons it is not included in the group of microorganisms. 
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Because virus need a living oraganism to reproduce so it is not include ib microorganisms
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The Brainliest Answer!
Viruses are non-living and they are live when they enter in other organism and as if they are non- living we can not consider virus as a microorganism
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