The first reason is that it needs to reach its destination... IIRC the JWST needed to accelerate itself to avoid damage on its initial orbital insertion (rather

The first reason is that it needs to reach its destination... IIRC the JWST needed to accelerate itself to avoid damage on its initial orbital insertion (rather than relying on the launch vehicle). It also needs to stop once it reaches its destination. Electric propulsion doesn't provide enough thrust for this particular mission. Electric is better suited for very slow acceleration buildups rather than quick corrections. It does actually have reaction wheels to control attitude without propellant, but it needs fuel to unload the momentum. It's possible the JWST could get into a situation where electricity wouldn't unload the momentum fast enough. https://en.wikipedia.org/wiki/Spacecraft_bus_(James_Webb_Space_Telescope)#Rocket_engines,_attitude_control https://en.wikipedia.org/wiki/Spacecraft_bus_(James_Webb_Spa... https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-momentum-management https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-m...