The mind of a goat is an amazing thing. They can see a hole in a fence from 20 yards, enlarge the hole with a tenacity that should put mammals with opposable thumbs to shame and climb obstacles that would challenge some college athletes, but even goats have their limitations, one of which is the ability to count beyond TWO. Two digits on their hoof, anything beyond that exceeds their mental capabilities. Such was the case with Ysis, poor girls couldn't count the third triplet to save her life. After two ate, she was baffled by the appearance of a third kid seeking her udder and would walk off, convinced someone was coming back for seconds.
Yucatan's issues are a little different. Yucatan had a single kid. Yucatan is a dairy goat in the truest sense. Her body understands that milk production is the primary purpose of her existence. Now, Argentina was a good sized kid at birth and ate heartily since the day she was born, but not even that appetite can begin to keep up with mother's milk production. Yucatan is now feeding her own kid, plus supplementing milk for the triplets who waddle away from nursing.
The other problem with a single kid is similar to the problem humans have with "only children". They are spoiled and life revolves around them. Argentina, for her whole 10 pounds of body weight believes that SHE is princess of the pasture, proceeding, with gutsy indifference to the dramatic size difference, to push around the January born Saanen and LaMancha kids. And, should anyone challenge her authority, she bounces back to mother who will, of course, protect her little darling. Twin kids would never stoop to such levels. They have siblings of equal size, strength and maternal affection who would quickly stop such prima donna nonsense.
God gave goats two teats, two hoof digits to count with and the brains to cope with twins - no more, no less. Hopefully the remaining 4 does to kid take heed and deliver twins!