The rugged A300-600 is still very popular with airlines like DHL, FEDEX, DHL and Air Hong Kong.
It´s low price, double ld2 lowerdeck, and the fact lots can still be converted makes developing the A330F premature.
For longer ranges probably bigger converted 747 are preferred.
Also lots of good MD11 are available for conversion.
The 332 is selling like hot cakes so no need to fill production slots with empty hull 332 freighters.
When 350 comes on I can see older 330/340 conversions..