Because the poem gives each founder the voice of a member of the rival order. In the Sun, Thomas Aquinas, a Dominican, spends canto XI telling the life of Francis, and in canto XII Bonaventure, a Franciscan, tells that of Dominic. The symmetry proves something no direct praise could prove: that the praise is free, because whoever gives it has nothing to gain from it.
Thomas tells Francis's life as a marriage to Poverty. He asks that whoever speaks of the place where Francis was born not say Assisi, which would say too little, but Orient. Poverty, widowed of her first husband, stayed for a thousand one hundred years and more despised and obscure, uninvited, until Francis came. Bonaventure describes Dominic as a lover of the Christian faith and a holy athlete, gentle with his own and cruel to his enemies.
Both praises end in criticism from within. Thomas says that Dominic's sheep, the farther they stray from the shepherd, the more they return to the fold empty of milk. Bonaventure says that the orbit at the top of the circle of Francis's order has been abandoned, and that there is mold where there used to be crust. Whoever accuses knows what the institution was at the beginning, and that is why the judgment carries authority.