Heathrow - Exeter is 180miles, so it is not that great a distance for visitors on an organized tour. And on the way to Devon/Cornwall, usually places like Bath, Salisbury or Wichester are taken in. If German tourists, probably more like visitors from the UK, would stay put in a place for a week or two, direct access via Newquay would probably be more important. But as they are most often travelling around anyway, the flexibility flights via LHR allow (and their lower costs) probably outweighs the benefits of direct flights into NQY for the vast majority of the clientele.