Since the L2 point is unstable, JWST needs engines to maintain its orbit. It uses mono-propellant engines which have given it a 5-year minimum lifespan.
Why weren't ion engines used instead? Wouldn't they provide much more Delta-v for the same mass of fuel?
Are they not reliable enough? Or is it just too much extra complexity?
Since the L2 point is unstable, JWST needs engines to maintain it's orbit. It uses mono-propellant engines which have given it a 10 year (maximum) lifespan.
The JWST uses bi-propellant engines (hydrazine and dinitrogen tetroxide) to perform midcourse corrections on the way to the Sun-Earth L2 pseudo-orbit, to perform orbit insertion into that pseudo-orbit, and to perform orbit maintenance maneuvers. It only uses monopropellant engines for coarse attitude maneuvers and reaction wheel desaturation.
The midcourse corrections, orbit insertion, and orbit maintenance maneuvers require a good amount of delta V that can be delivered quickly. Even a monopropellant hydrazine engine wouldn't suffice. Adding monopropellant engines and a bit more hydrazine for use by those monopropellant engines apparently made more sense in the initial design than did adding a whole new set of tanks and thrusters for ion engines.
While some initial design decisions can be changed, thrusters are amongst the many items on a spacecraft that are pretty much frozen from day one. Moreover, that initial design occurred about 20 years ago, when ion thrusters were just past the experimental stage. And they certainly weren't going to switch to ion thrusters. Deep Space 1 was launched in December 1998, and it worked, kinda-sorta. DS1 had significant problems and was not a complete success. It's ion thrusters didn't work at first and the vehicle performed poorly on some planned fly-bys.
It is possible in another world that the designers of the JWST could have modified their spacecraft 20 some years after the initial design to use ion thrusters for some aspect of vehicle control. Then again, they could have also redesigned the vehicle to use more modern computers, more modern solar arrays, etc. They didn't do so in part due to the immense amount of conservatism that is baked into spacecraft design.
@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.
It says:
...and orbit maintenance maneuvers require a good amount of delta V that can be delivered quickly.
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.
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.
