First of all, let us note that not all Republicans were called “radicals.” Only some of them were. Other Republicans were not radical. For example, Abraham Lincoln would not have been seen as a radical. To most historians, Hiram Revels does not count as a radical either.
At this time, the radicals were those people who wanted to treat the South harshly. They wanted to punish the South for having brought about the Civil War. Therefore, they advocated things like military rule of the South and stringent rules on when ex-Confederates could have their rights returned to them. Revels did not support these ideas.
There were times when Revels did side with the Radical Republicans. For example, he opposed the idea of having segregated schools in the District of Columbia. However, on many important issues, he was much more sympathetic to the South. One major instance of this was his support for the idea of allowing ex-Confederates to regain their civil rights simply by swearing loyalty to the United States. Because Revels supported policies such as this one, he is not typically seen as a radical.