Why Are There Thirteen Principles of Faith?:
It could be that the Rambam was hinting to the concept of the 13 attributes of compassion of G-d. But we must understand that these 13 principles are not exclusive - there are plenty of other principles mentioned by the Rambam in other places. There are several examples of other principles that are core to the Torah. One example of where he mentions other principles is when he discusses about a righteous convert, because the convert must be taught the principles of the religion.