I looked up the answer on the JW website. It states that they follow Acts 15;20 and Acts 21;25 that essentially states “Christians must abstain from blood”. From the article, I got the impression the belief is that only God gives blood and man cannot. They do accept blood alternatives.
What is strange is that those verses also say to refrain from eating the fat of animals and I don’t see any Jehovah’s Witnesses doing that!