Dear Chris,
I am not sure that this is so, I can think of many ancient observatories in the northern hemisphere - New Grange in Ireland, Stonehenge in England, Observatories where the Chinese emperor worshiped the Sun, the Ise Shrine in Japan, Medicine wheels (many hundreds of them) in the North American plains and Canada, Cahokia in current St. Louis, MO, the Pueblo and Navajo sites in the South west (many dozens of them) and many ancient sites in Mexico (most of which is in the northern hemisphere).
Perhaps in Europe, because of the fast changing landscape (socio-political) where phases of construction in cities happened many times, we don't find ancient sites where they once were.
Isabel Hawkins