The fact is that the Watchtower leaders need always to impose a scandalous doctrine. And as Gruss said , the blood ban was intended to convince the membership they were being "persecuted" and "suffering for righteousness sake," a sure sign they were "in the truth."
This fits the model of "strictness" depicted by authors Stark and Iannaccone in their 1997 article Why the Jehovah's Witnesses Grow so Rapidly: a Theoretical Application, published in Journal of Contemporary Religion (1997). In their presentation Watchtower's blood taboo is pointed out as one instance of this phenomena.