Not that I have any special insight or sit on the committee responsible for determining the allowable stresses, but the 0.3 factor is probably used to take into account the "crack initiator" present in the root of the fillet weld and partial joint penetration groove welds.
The incomplete fusion present in the root of the fillet weld, where the butting and nonbutting members are joined, acts as a notched stress riser. The reduced allowable stress is the accepted manner used to accommodate the stress riser.
Where did it come from, your guess is as good as mine.
Best regards - Al