Because, along with Dutch tulips and the South Sea Company, Beanie Babies are often used as an example of an economic bubble by people ignorant of the fact that the causes of the rise and fall of the value of these assets is well understood, and was predictable (to some extent) even at the time. Furthermore, the circumstances of these cases are all very different from each other and from Bitcoin, and any decent economist would laugh at the suggestion that they're comparable.