JWST is solar powered. Sun-Earth L2 is in permanent eclipse and may have unstable lighting or heating conditions. A halo orbit about L2 is extraordinarily boring and always in full sunlight.
Maintaining L2 or an L2 halo requires propellant.
Maintaining L2 or an L2 halo requires propellant.
(Edit below, to answer my own question via wikipedia)
> The telescope will circle about the Sun-Earth L2 point in a halo orbit, which will be inclined with respect to the ecliptic, have a radius varying between about 250,000 km (160,000 mi) and 832,000 km (517,000 mi), and take about half a year to complete.