The primary reason President Lincoln was unable to persuade Congress to accept his Reconstruction plan was due to his assassination in April 1865. This tragic event cut short his leadership and the ability to fully implement or advocate for his vision for Reconstruction.
Lincoln's plan, known as the Ten Percent Plan, aimed for a relatively lenient approach to reintegrating the Southern states back into the Union, requiring only 10% of voters in a state to take an oath of allegiance to the Union for that state to be readmitted. However, many in Congress, especially the Radical Republicans, felt that Lincoln's plan was too forgiving and did not adequately protect the rights of freed slaves or ensure a stronger commitment to rebuilding the South's political and social structures.
While the Freedmen's Bureau did play a role in Reconstruction, the primary contention was between Lincoln’s vision and the more stringent demands of Congress. After Lincoln's death, Congress took a much harsher stance on Reconstruction, culminating in the Reconstruction Acts of 1867, which imposed military rule in the South and set stricter requirements for rejoining the Union.
In summary, while there were disagreements over the nature of Reconstruction, Lincoln's assassination ultimately prevented him from persuading Congress to adopt his plan.