I would say it was partly a failure of will, but also partly strength of will that it failed. In terms of the northern population, many people outside of Congress were sick of all things Civil War, and were, themselves, racist, and therefore unconcerned about whether or not civil rights were actually a reality for freedmen. They were lured and distracted by the possibilities in the West, and consequently, their failure of will did lead directly to the failure of Reconstruction.
On the other hand, southern will against the Yankee occupiers and to keep their way of life and economic structure intact was very strong, and also directly contributed to Reconstruction policies failing almost universally.
So you're asking about why Reconstruction failed, right? If so, I would vote for "failure of will."
Reconstruction failed largely because Northerners did not really care about the freed slaves. They did not care about them enough to continue to subject the South to military government.
During most of Reconstruction, the South was ruled by a military government imposed by the North. This is something that is completely antidemocratic and unAmerican. In order to do this to fellow Americans, the Northerners needed to be strongly motivated. But the Northerners were not very motivated. They did not think that the cause of black rights was a good enough reason to continue to oppress the South. Therefore, they gave up on Reconstruction.
For this reason, I would argue that the failure of Reconstruction was caused by a failure of will.