It was found centuries ago that these materials: wool cloth and paraffin wax, glass rod and silk cloth when rubbed against each other attracted one another. While two glass rods when rubbed against their respective silk cloths repelled each other and the same befell two pieces of paraffin wax. These forces of attraction and repulsion also manifested in the pieces of cloths that were used for rubbing. Hence, it was found that any pair of materials that demonstrated properties of attraction and repulsion after begin rubbed together could be classified into one of two categories: 1) Attracted to wax and repelled by glass 2) Repelled by glass and attracted to wax. Early experimenters suggested that there might have been fluids that transferred from one material to another that caused the attractive and repulsive forces. Charles Dufay suggested that there are exactly two types of changes that befell the pair of certain materials when rubbed together. This suggestion was based on the fact that there exist two types of forces of attraction and repulsion. The hypothetical fluid transfer became known as charge. On the contrary, Benjamin Franklin suggested that there was only one fluid exchanged between the two objects when rubbed together and that the two charges were nothing more than the excess or deficiency of this one fluid. He performed an experiment on wax and wool, and suggested that the wool takes some of the fluid from wax and this imbalance causes the attractive force because the fluid tends to be re-balanced. Postulating the existence of a single fluid that was either gained or lost through rubbing accounted best for the observed behavior: that all these materials fell neatly into one of two categories when rubbed, and most importantly, that the two active materials rubbed against each other always feel into opposing categories as evidenced by their invariable attraction to one another. In other words there was never a time where two materials rubbed against each other both became either positive or negative.
I know these are the old concepts, but ignoring the modern concepts My question is that, why was Franklin's Benjamin suggestion the best explanation to the observed behavior that all the materials fell into one of the two categories when rubbed? The suggestion that there were two types of fluids that were transferred from one material to another could also explain the same thing.