[Checkyersix]

It could be more a matter of the intended target. 2 x 500lb bombs would give you more shrapnel than 1 x 1000lb (because of the higher surface area), which would be more damaging against soft targets like trucks. A 1000lb bomb would be more useful against fortifications or armored targets. Availability was probably a factor as well - there might have just been more 500lb bombs around.