Bit of a rambling waffle, but my gut-feeling is simply that previous burials were left alone because it was viewed as either morally wrong or likely to incur the displeasure of the gods to disturb them,
To throw a spanner in the works, how would Gib Hill fit into that, with the round barrow having being built on top of the former long barrow. ?
Some may say it's enhancement, others desecration of an ancestral grave.