I know nothing about hieroglyphics, so here's a complete shot in the dark for an answer, it is probably because there are different types of love. The love you feel for your boyfriend, is different than the love you feel for your brother, or mother, or father. And then there is the love we are in denial about.
I think there are six hieroglyphic for love because they are used to describe the kind of love you feel towards someone. Using one hieroglyphic could be a bit monotonous in showing affection to someone you really love.