The A300s pre-date the A330s by several years.
Qatar Airways first introduced the A300 in 1997 to allow retirement of the B747-100s that had been used on European flights. Not only were the 747s too big for Qatar at the time, they were getting expensive to maintain...
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Photo © A J Best
The first A330 was not delivered to Qatar until 2002, I expect in the late 1990s that the A300-600R was probably the most cost-effective type for Qarar - at the time they were still only a small airline.
Qatar Airways are in the process of retiring 2 A300s from passenger service and converting them to freighters, I would expect the rest of the passenger A300 fleet to follow eventually.