They have a relatively high level of zeaxanthin compared to other fruits and vegetables, which is more difficult to find naturally than other nutrients. (dietselfhelp.com)
In addition, yolks also contain zeaxanthin, an antioxidant that protects your eyes from the harmful ultraviolet radiation. (fitnessandpower.com)
Since cauliflower leaves (and perhaps broccoli leaves) are higher in some nutrients than the other cruciferous, it's possible those are higher in zeaxanthin as well. (nutritionfacts.org)