They may have volunteered, many were very eager to show they could fight, and wanted to fight the Rebs they hated.
Their command may have been larger, since there were no mixed units, one or the other commands had to man the wall.
The white troops may not have been combat ready for some reason and were held in reserve.
Black troops were very aware of their no-quarter status and wanted to fight first to determine their own fates rather than rely on white troops that might surrender them.
In fact, the reason both offers of surrender were refused is that the blacks knew what their fate would be and strongly objected to being taken prisoner, they were right.
A reasonable speculation, but speculation nonetheless.
The Union was waiting on reinforcements from the gun boats. The plan was to cover a Federal retreat with cannister if need be. Forrest's troopers moved so fast the gunboats could not fire without hitting the retreating bluecoats.
Forrest was known for treating prisoners fairly and went even farther in the next battle to prove again they would receive fair treatment.