Largely because of the breadth of his interests -- artistic, intellectual, and scientific -- which moved well beyond the standard religious worldview of the pre-Renaissance era and involved a wide variety of areas and new fields of inquiry.
And when we speak of a "Renaissance man" today, we usually refer to someone who, like Leonardo, is seriously interested and well-versed in several areas.
It's true that you won't understand the term "Renaissance Man" (or Woman) if you don't know what the Renaissance was. Surely you've been studying it in school? Reading books on the subject? I've given you some sites, but you are better off with books.
