I have a guess, not more:
It's a widebody, it has a lot of power, so it's a good hauler for heavy loads, it is and was available in sufficient numbers at affordable prices.
As we see a lot of the A300 used as freighters a big number are old first generation A300 aircraft, still with the three-men-flightdeck. These aircraft became available to the freighter market well before affordable B767 were available. As most freighter aircraft used to be former pax aircraft this may also play a role in the success of the A300 as a freighter.
From what I see, it's a good aircreft for hauling freight on short to medium long sectors offereing sufficient load capability paired with good performance and the aircraft were available for freighter modification as the first widebody aircraft not meant to be a long haule aircraft.
All above is just my personal guess and I must say I have never flown one of the A300.
