It is a convention. You can work with either positive or negative current, it simply does not matter. The Navy taught me negative current flow, other places teach positive current flow, it really does not make any difference...
Other then it simply makes more sense to speak of the flow of electrons as the current.
Yes, I am VERY aware that in ionic solutions positive ions can be the current carrier! Last I checked wires are NOT ionic solutions so that is a red herring when discussing current flow in electronic devices.