"Because the Governing Body has told them they shouldn't. Disobedience means disfellowshipping, with both long term (loss of paradise earth hope) and short term (loss of all JW family & friends) consequences."
This is the right answer. Most of the JWs eyes glaze over when the blood issue is discussed at the KH. They don't fully understand the scriptures, the medical terminology, or the science involved, and they're not willing to research it. They are not capable of making an informed decision on the matter because the only thing they've heard or read is the watchtower's convoluted reasoning. Most don't even know what fractions are. My mom used to avoid putting fertilizer on her rose bushes because it might have blood in it.
They refuse blood because the cult says so.
W