Answer :
Leeuwenhoek is not a creator of microscopes, but he has advanced knowledge than any other scientist. He concentrated on the study of different species, and he had excellent experience in describing the human natures together with other tiny tissues in the body. He has the capability of discovering the smallest things that can only be viewed through the microscope and in another human, can see them.
The experience gave him the opportunity of creating a small microscope because of his advanced knowledge.