Because they are uncertain as to where they are and where they are going?
Well, from a charge point of view it's arbitrary - we could easily call electrons positive and protons negative and the show will go on. But historically, current was observed before electrons were known about. Current flows from higher to lower potentials, and it probably would not have occurred to anyone to call this 'negative current' in absence of any corresponding current flowing the other way, so this was chosen as positive. Electrons flow in the opposite direction to conventional current flow, so are negative.