Why was the Republican
plan for Reconstruction called "Radical
Reconstruction"?

Because of the radical changes that they sought to implement in society.

Explanation:

This change primarily concerned the African-American population. Republicans intended to give this section of the community much higher rights than they had up until now. In the first place, this meant voting rights (men only). To further improve the position of this section of the population, efforts were made to give them the right to education, private property, and even the freedom to political engagement. Following the above, in the first months of 1868, America had 700,000 new eligible voters.