As stated in Principles of Plasma Discharges and Materials Processing, by M. A. Lieberman and A. J. Lichtenberg, about capacitively coupled plasmas:
(...) the light and heavy charged particles in low-pressure processing discharges are almost never in thermal equilibrium, either between themselves or with their surroundings. Because these discharges are electrically driven and are weakly ionized, the applied power preferentially heats the mobile electrons, while the heavy ions efficiently exchange energy by collisions with the background gas.
See also this answer to a related question. This difference in coupling strength is given as a reason for the difference between electron temperature and ion temperature:
However, I don't quite see the reason for this difference in coupling strength. Why is it that an oscillating electric field transfers energy less efficiently to ions than to electrons?