Because some humans are inherently inclined to possess certain negative traits, such as selfishness and a willingness to unlawfully consume people's wealth, a great rule was established by the Messenger of God (peace and blessings be upon him and his family) to establish rights and prevent their loss. This principle is stated in his saying (peace and blessings be upon him): "If people were given what they claim, some would claim the wealth and blood of others. But the burden of proof is on the one who claims, and the oath is on the one who denies." The burden of proof here is the witnesses. The wisdom behind the burden of proof being on the plaintiff is that his side is weak because he claimed contrary to the original principle, so he is obligated to provide proof. The defendant's side is strong because a person's conscience is innocent unless he is preoccupied with something, in which case he is required to swear an oath. This is a weak argument, but it prevents harm from its owner and brings him benefit.
Fayyad et al. (Sat,) studied this question.