It's mainly cultural bias, established early on when amateurs were publicising what they thought were the most notable contributions of different past cultures. There are fascinating megalithic structures in Italy (see the Stone Pages on Sardinia and Puglia) but the place is just so chock full of Roman and Renaissance remains that obviously had a great influence on subsequent history the stones were ignored. In France the great cathedrals were easy to study and admire. Great Britain, which you have to admit was a bit of a backwater--at least as far as influencing European history before, say, Elizabeth and the Armada--didn't have this kind of "glorious" DEEP past, but it had Stonehenge, unique in Europe.

Once these biases get established, the limited money available for historians and archaeologists follows the already beaten paths. Students argue with or idolize professors, perpetuating their interests. Committees don't like hearing about anything radically different: they don't know how to evaluate it. It's then up to the new amateurs, such as you folks, to collect enough basic information about, and demonstrate enough interest in, new stuff to convince professionals there's a market for their books and lectures, potential students for the mavericks among them, etc.

It's not a great system, but its inherent conservatism fosters depth of interest. Given limited funds, that may be more productive than shallow surveys of everything. Particular expertise can be judiciously applied to new situations, but attempting to collect information about everything all at once will never result in the depth of knowledge we now have, or are getting, about a few places.

Just my take on it all.