Why do they say to keep wine out of sunlight?
Sunlight is art's greatest foe, and it is also wine's greatest foe.
Direct light can break the molecules in a wine, can render a rosé colorless.
Sunlight also causes heat. Heat will damage the wine by destroying/changing some molecules, but also pushing/pulling air and/or the cork which increases the chance of oxydation.
Keep wine in the dark.