The main constraint for JWST is that it needs to be able to block all IR radiation from the Sun/Earth/Moon in order to pick out the faint IR signal it's looking

The main constraint for JWST is that it needs to be able to block all IR radiation from the Sun/Earth/Moon in order to pick out the faint IR signal it's looking for (from red-shifted light from the very early universe). By orbiting L2, it keeps Sun/Earth/Moon all on one side of the spacecraft, which it's then able to block with its sunshield.