Why is leonardo da vinci considered a renaissance man?

Because he typified the idea of the Renaissance.

The term "Renaissance Man" refers to one who was capable of excelling at a wide variety of activities.

Leonardo was an accomplished scientist, architect, engineer, painter, designer, physiologist, etc. Truly he was not simply a man of the Renaissance, he was the prototype of the Renaissance Man.

Leo epitomized the idea of Renaissance. That term means a dude who is capable of excelling quite well at many stuff. For Leo, artwork, science, engineering and math.