Flexibility and speed. Arbitrators tend to be more flexible than Courts. Avoidance of precedent. Arbitrations set no precedents. Federal law preempts almost all contrary law.
Most people are happier with the results of arbitration.
There is very little meaningful appeal or review. In some circuits, you can appeal from a manifest disregard of the law, in others you can not. Limited discovery and proceedings. The procedure is restrictive in order to reduce costs. Absence of evidentiary rules.