renaissance art is focused on giving a rather humanistic view of christianity. if you would observe art during the medieval period, images of jesus, the saints and all other deities is are not rendered in a realistic, humanistic manner, but rather to semi-abstract forms. the reason for this would be that such concepts are inconceivable to have human-like images simply because they are not mortal beings (the reason may be similar as to why in Islam, Muhammad does not have any depiction). rennaissance changed this idea, specially in its philosophy of gloryfying achievemnts of the human mind. as for davinci and the last supper, i think his objective is to depict the eventm as realistic and humanistic as possible. other than that, you may ask his patron.