My feeling is that this was a transplantation of people from Wales to Wessex who brought their local land with them (kind of like Dracula needing the earth of Transylvania in his coffin).

The southwest walians recreated their original landscape's sacred power 150 miles away in Stonehenge by arranging the bluestones in the same relative positions as the outcrops from which they had been quarried.

As to why the people made the move in the first place, and why Wessex was the destination.... if I could answer that (without being laughed at) I would :-)
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simon