Because the body is primed to respond to the sugar and reject other foods in favor of it.
Now, way back in caveman times, that was great. Sugar (glucose) is nature's energy storage, and when our hunter/gatherer ancestors found a rare taste of fruit, it made sense for them to grab it before any other animal did. In nature, fruit rarely lasts long on the vine, and the early humans who craved sweetness more got more vitamin C -- more of all the nutrients in fruit.
But, evolution didn't anticipate Nabisco. What refined sugar / HFSC does is it hijacks an evolutionary advantage and makes it toxic. If you prime the palate with refined sugar (and yes, brown sugar IS refined sugar) too young, your daughter will reject every other food there is. I know, because I know and have worked with plenty of children whose lovely, wonderful mothers just wanted to give their cute little baby/toddler a "treat."
Please don't do it. It activates the center of the brain that gets activated in addictions. It leads to a lifetime of health disasters. Think of it like, you wouldn't let a 5-year-old boy drive a car, right, even if he loved cars and would have soooo much fun? No, you'd wait until he was 16 and was bringing home decent report cards.
Save this treat until she's old enough to handle it. Which is not now.