The answer is not clear-cut, but it is likely a combination of factors. One factor is that President Lincoln's plan was seen by some members of Congress as too lenient towards the former Confederate states. Additionally, Lincoln's assassination in 1865 meant that he was unable to enforce his plan, and his successor, Andrew Johnson, had his own, more controversial, approach to Reconstruction. Finally, there was significant disagreement among Republicans in Congress about the best path forward for Reconstruction. Some advocated for a more punitive approach, while others favored a more conciliatory one.