Because it oversimplifies the period to sound like nothing but misery and death, when in fact one region of the world went through a crisis after the collapse of its regional hegemon, and then after several decades of chaos new nations started to emerge.

The Lombards and Merovingians, for example, would have been very surprised to learn that they were living in a "dark age", to say nothing of Charlemagne.

Part of the reason is that it only refers to western Europe but people use it for the entire time period when there were so many other things going on in the world, from African kingdoms to the Gupta empire (considered a golden age in India) to the Tang and Song dynasties (considered a golden age in China) to the Byzantines, to the rise of Islamic empires, to the Sassanids in Persia, to the beginnings of Classical Japan and so on and so forth. It's incredibly Eurocentric and when applied to the world can actually be the opposite since it was a high point for many civilizations.

Even in western Europe, it's quite a judgemental term. Rome being more advanced and its fall being the tragedy of the millennium is quite subjective. I'm sure kingdoms like those of the Lombards or Visigoths etc would consider themselves quite cultured.