I would guess that a large part of it is that it's the oldest period of time for which we have significant details, and a strong association with the people the

I would guess that a large part of it is that it's the oldest period of time for which we have significant details, and a strong association with the people there as our ancestors (in the respective countries where such fairs are prevalent). We have much less idea of what ancient Roman or Celtic or Norse or Thracian etc day to day life actually looked like, and to the extent that we do know, it's much weirder and more foreign.