Answer:
Radical Republicans disagreed with this Presidential Proclamation because they thought that President Johnson is being too merciful towards the Southerners.
Explanation:
On May 29, 1865, President Andrew Johnson declared a Presidential Proclamation under which, with the purpose not to punish Southerners and quickly reunite the States, he granted political rights to those who swore their faithfulness towards the States.
In the viewpoint of Radical Republicans, this Presidential Proclamation was indulgent towards the Southerners who started the war. According to these Radical Republicans, these Southerners needed to be punished harshly.
Therefore, the reason why Radical Republicans disagreed with this proclamation was because they considered that President Johnson was being too merciful towards the Southerners.