why is it that if one parent is a convert it is permitted but if both are it is forbidden

ב"ה Shalom The law in the Shulchan Aruch is derived from the Talmud in Kiddushin 78a. The Rabbis in the Talmud there, expound the verse {Yechezkel 44:22) which says that Cohanim should not take for themselves as wives a widowed or divorced woman , however they make take for a wife a woman "Mezera Yisraael", which means that the woman descended from Jewish seed. Rav Yehuda, explains that the meaning of the verse is that the woman must descend from both parents who were born Jewish, while Rav Eliezer b. Yakov expounded the verse that "Mezera Yisraael" means even if one parent was born Jewish, a Cohen may marry her. The Shulchan Aruch followed the opinion of Rav Eliezer b. Yakov, whose opinion was that even if one parent was born Jewish, a cohen may marry her. But according to both, if both parents were not born Jewish a cohen may not marry her because this is what is indicated in the verse. And as I have explained in previous answers to similar questions, the Torah restricted who a cohen may marry in order to preserve the special status of the cohen within the Jewish people. All the best