The answer is "follow the money".

Oneworld although it is a legal entity is not really a commercial entity in the same way an airline is. Each FFP has it's quirks and benefits, and typically there is something to reward frequent flyers to focus their flying activity on a selection of airlines, that drives or influences spend.

With a common OneWorld FFP the member airlines of OW would loose that flexibility to drive or influence spend, and not directly control the money from the FFP scheme.