I would suggest reading up on the following support posts:
Although they don’t directly address the allocation method used for BFS or FA it can add a bit more to the understanding of the subject.
However to answer your question, allocation of impacts for BFS & FA is done based on economic value. It is already quite popular in the building industry to add blast furnace slag to concrete as it increases the durability of the concrete. This demand increases its economic value. Fly ash on the other hand is still mostly treated as a waste product and has no steady demand for it to be reused even though it does improve the quality of concrete when used. Therefore at this point in time, in terms of environmental allocation, there is more environmental incentive/benefit to reuse FA over BFS in concrete reduction measures.