Gas is continually being lost by Io, ionized, and added to the Io plasma torus. Why doesn't the plasma torus become more and more massive? Because plasma is also constantly being lost, at about the same rate (on the average) as it is added by Io. It is believed to be lost by being flung outward by centrifugal force, after the manner of whipped cream flying off the egg-beater blades.
Why does the Io plasma torus lose mass at the same rate that it acquires new mass? Is this a coincidence?
Evidently, the more mass the Io plasma torus has, the faster it loses it. Therefore, whatever the supply rate, the loss rate will increase or decrease to balance the supply. The reason for this behavior is still not well understood.
Contrariwise, why doesn't the whole Io plasma torus fly away immediately, instead of hanging around and leaking away slowly? The answer to this question is also unknown, but may be related to the stirring that occurs as new mass is added from Io. When stirred, the plasma torus has more trouble forming blobs that drip away from the confining magnetic field lines.