The A330-200 and -300 offer a higher flexibility than the B767-300/400 do, that´s at least my opinion. The A330 is slightly larger the the B764, as it seems at the same or even lower costs.
ATA´s markets are comparable to the European charter markets (low-yield), here most airlines which had to choose a DC-10/L-1011 replacement have opted in favour of the A330-200/300, only a very limited number has taken the B767-300ER.
For many airlines cargo is a good extra business and we all know that the B767 isn´t exactly a good plane for carring belly cargo due to the different container size, here the A330 has the advantage.
Besides one has to ask: is a single-manufaturer fleet really the best thing you can have? I honestly don´t know it but I don´t think so, it makes you too depended on another company.