@DavidHammen's answer goes a long way towards answering, especially in that the space telescope's bus was finalized quite a long time ago when ion propulsion was much less a proven long-term reliable technology.
...and orbit maintenance maneuvers require a good amount of delta V that can be delivered quickly.
They may not be strong enough to recover from a short interruption in station keeping
Coms satellites in GEO are now becoming "all electric" for station keeping and many are doing GTO to GEO with electric propulsion as well.
So we might ask if the space telescope were to be designed today if it would use electric propulsion for station keeping or not.
That the space telescope's halo orbit is exponentially unstable and if a few of the bi-monthly station keeping maneuvers are missed for some reason it may get harder and harder for low thrust to be able to bring it back to station.
Once the acceleration away from the halo orbit along the unstable manifold exceeds the acceleration provided by electric thrusters (which is always pretty darn small) all is lost and it will spiral out towards a heliocentric orbit.
There is no analogy to this exponential instability in the station keeping of comms satellites in GEO. These are very well gravitationally bound to Earth, and the won't wander off into a heliocentric orbit if left unattended.
But wait, there's more (weight)! (potentially)
The question considers the weight of ion propulsion propellant being lower than that of conventional thruster propellants, but ion thrusters have heavy magnets and power supplies to produce the plasma.
It's very possible that once the full electric propulsion system is designed and weighed, one strong enough to return JWST to its halo orbit if a few bi-monthly station keeping iterations were missed, that it doesn't end up any lighter.