The correct answer to this open question is the following.
The major cause of the end of Reconstruction was the informal negotiations between Democrats and Republicans to declare the winner of the elections of teh 1876 Presidential election. They negotiated and Democrats agreed to declare Rutherford B. Hayes with the following conditions.
Republicans accepted to sign the Compromise of 1877, in which they agreed on withdrawing the federal troops established in the southern states that were there to support Reconstruction. So without the presence and support of the federal troops, Reconstruction saw its last days and finally ended in 1877.