At the time of Ben Franklin electricity was know to be composed of two kinds of charge, which Franklin called positive and negative.  Since charge could move from object to object it was assummed to be a kind of fluid.  Scientists thought that two charges meant two kinds of fluid, positive fluid  and negative fluid.  It wasn't until the late 1800's that scientists discoved that the positive charge is carried by particles that are much heavier (almost 2000 times) than the negative charge, and that the positive charge is concentrated in the center of the atoms.  So the only fluid was the flowof negatively charged electrons.  However, by that time over a century of scientific work defined the current direction as the flow of positive charge.  So they decided to continue writing equations as if the charge were positive knowing that what is physically happening was the flow of negative charge in the opposite direction.