As for reusing good ground, in some parts of the world there are tells, resulting from layers of human habitation:
The tell of Hisarlik is up to 9 Troys:
http://cerhas.uc.edu/troy/index.html
People have reused/repurposed holy sites for a long time; the Parthenon, in just one instance of many, functioned as the site of a church and later a mosque:
http://www.pbs.org/wgbh/nova/parthenon/time-nf.html
In Europe, though, I wonder if it was a matter of reusing noted holy ground of some kind, but also they’d run out of decent real estate on the main roads, pilgrimages having long been the local chamber of commerce’s dream, as well as an ancient and respected form of religious activity.
Makes you wonder why obviously important sites like Gobleki Tepe were never built over:
