I think it is because parsley is too high in calcium. There is some really good information on veggies on the guinea lynx site, including a comparison chart: https://www.guinealynx.info/nutrition.html#veg
Cilantro and parsley are NOT the same thing. Cilantro is the bushy/leafy part of the corriander plant. Parsley is a separate spice all it's own. Parsley is high in calcium, Cilantro is not.
You can also tell by the smell. Cilantro has a much stronger smell then parsley.
Cilantro has bigger, flat leaves. It is usually found in the produce section right near the parsley. It is also called Chinease Parsley. It is the leaves of the corriander plant.
My guys love cilantro, expessically when I rinse it with water. They get some every day with their salad. Also I get it very cheap between 5-10 bushels for a buck.
Percy seems to like most anything with leaves...except kale and spinach. Cilantro is an absolute favorite though, and it's always cheaper than regular or italian flat leaf parsley around here. He likes the tops of root vegetables as well. Recycling at it's best when I cut up radishes and carrots for the rest of my family's salad!