"the problem of the A300 was it’s limited range. For the operators the 767-300 made more sense."
But the A300 was 10 years ahead of even the basic 767 in service. One of the reasons for the slow take-up of course was a lot of the '70's airlines and economies were in recession - especially in the UK. Shelling out for a big new airliner when you could keep that BAC111 running wasn't attractive