The incident that you are talking about came in the 1875 election. In order to understand why this would be, think about how party politics were at that time. Today, African Americans are almost all Democrats. Back then, by contrast, African Americans were practically all Republican and the Democratic Party was associated with racism and racist policies.
At this time in history, the Democratic Party was associated with the Confederate States and the Republican Party was associated with the Union and the victory in the war. It was the Republicans who were running Reconstruction in the South. Because of this, the whites in the South almost uniformly hated the Republicans and black Southerners supported that party. Moreover, the Democratic Party was connected to the violence that was being perpetrated against black Southerners. The violence was being used as a way to stop them from voting.
In this sort of a situation, it is not surprising that African Americans in Mississippi would be angry if Revels supported the Democratic Party. When he denied that any intimidation and violence were happening, he would have appeared to be selling out the black community in order to curry favor with the Democrats. This would naturally have made them stop supporting him.