Why were the radical republicans angered by Lincoln's 10% plan?

Lincoln's "Ten percent plan" was a proposal made during the Civil War that set out how post-war Reconstruction was to take place. Its gradual approach to the matter had all the hallmarks of Lincoln's compromising nature. The President knew that, after this bitter conflict in which the country was still engaged, there would need to be some kind of accommodation with the South. The question was what lengths the Federal government should go in order to achieve this goal.
To that end, the "Ten percent plan" proposed that a state in rebellion against the US Federal government could be readmitted to the Union if ten percent of its 1860 vote count pledged allegiance to the United States and agreed to abide by the Emancipation Proclamation. The overall tone of the plan was moderate in keeping with its stated aim of bringing the war to a speedy conclusion after two years of increasingly savage conflict.
However, Radical Republicans in Congress were outraged at the plan. They thought that it treated the rebel states too leniently. As far as they were concerned, the Confederate states had started the war, committing an act of open rebellion against the legally constituted authorities. As such, they should be punished for their rank treachery. Lincoln's proposals to re-integrate them back into the Union were seen by Radical Republicans as almost a reward for sedition, potentially encouraging future acts of rebellion.
Also, the plan's changes had no real depth to them. The Radical Republicans wanted a root and branch reform of all aspects of Southern politics and society. It was only by doing this, they argued, that the South could be punished for its misdeeds as well as making sure that there'd be no future uprising. The "Ten Percent Plan" didn't even begin to address these concerns, and placed too much trust in the sincerity of a loyalty pledge and mere lip service to the principle of emancipation. 
https://www.history.com/this-day-in-history/lincoln-issues-proclamation-of-amnesty-and-reconstruction