Because it is an infrared telescope, and in earth orbit there are three very bright infrared sources that would mess up the instruments or heat up the telescope too much if they ever shone on the sensitive parts. (Sun, Moon and Earth.) When the telescope is placed in the Sun-Earth L2, all three sources are always very close to each other in the sky, and you can shield against all three with a single directional sunshield.

The coldhead of one of these sensors (MIRI) functions optimally at temperatures less than 7˚K (i.e. 7˚ above absolute 0). The other sensors run at temperatures under 40˚K.

For reference, the melting point of nitrogen is, 63˚K.

Any sunlight at all would overwhelm this sensor and severely reduce the operational lifetime (and results!) of the telescope.

At L2, the telescope has the sun and the Earth behind a layered sunshield. It's easier to maintain position there and to have both objects always in the sol-ward direction while looking outwards.