“This
is a lasting ordinance for the generations to come, wherever you live: You must
not eat any fat or any blood.” Leviticus 3:17
“Anyone who eats blood must
be cut off from their people.” Leviticus 7:27
People
drank blood back then for other religious rituals, believing it gave them power
and life (like vampires in stories today).
Also,
God understands that certain diseases could be transferred through the blood
unless it was cooking thoroughly. For those
reasons he wanted to make sure the people ate cleanly.