Why, Oh Why?
Many children are astoundingly picky. I'm glad I'm not a pediatrician, because now I can spout off a lot of theories about why, and no one can hold me to them, though I'm sure I'm right.

It seems to me that when babies become toddlers, the household feeding dynamic changes dramatically. Parents go from spoon-feeding their child and applauding every bite ("She ate all her strained beets today!") to pulling the high chair up to the family table and expecting their toddler to eat along with them — to eat when and what the rest of the family is eating. Accordingly, parents start urging their kids to eat more foods, in more varieties, lots faster — which, naturally, the toddler resists.

Added to that is the fact that for some reason, babies' taste buds seem to change and become much more selective as they approach toddlerhood. I suspect this trait evolved from caveman days: A mobile toddler could have wandered away from her parents and eaten potentially dangerous food — deadly roots and berries and so on — instead of sitting still and eating what her parents fed her. Whatever the reason, the dreaded taste metamorphosis happens as kids approach the 18-month stage, and that beet-eater is no more.