Isn't 11-trans-retinal more stable than 11-cis-retinal?
Cis-retinal is less stable than the trans isomer.  The carotenoids ingested by the body are all trans.  Enzymes in the body convert them to trans-retinoic acid, then turn them into a fatty ester which is then converted by the enzyme retinoid isomerohydrolase to cis-retinal.  The higher energy state of the cis isomer makes it prone to photoisomerization back to the lower energy trans isomer.