What sunlight actually does
Ultraviolet light attacks the outer layer first. Nylon-jacketed THHN chalks and cracks; PVC jackets stiffen and discolour; rubber portable cord develops a hard skin. Cross-linked polyethylene used in XHHW and MV cable resists UV better, which is why those reels often survive a yard that ruins building wire.
Heat and thermal cycling
A steel reel in direct Texas sun gets hot enough to soften some jackets at the point of contact. Standing a reel on its side in the sun exposes a large flat face to that heat, and the cable in contact with the flange takes the worst of it.
Water and the ground
Reels stored on bare ground wick moisture through the wood and into the cable at cut ends. Set them on dunnage or gravel pads, keep them upright, and never store a reel with an open cut end in a puddle.
Storage that protects value
Shade, airflow, dunnage and end caps cover most of it. If outdoor storage is unavoidable, keep the reels upright, block them against rolling, and photograph them where they sit — a buyer can see sun damage in a photograph and will price it honestly.