Because they are "washed in the blood" covenant of the Lamb. Matthew 26: 27&28 makes it clear that the only access to the blood that washes us clean is through the New Covenant.
Doesn't logic tell you that nothing can be washed and cleansed in blood? Blood poured out of a living thing stains, ruins and putrefies.
Try and step out of the religious indoctrination for a moment and see that this is an illogical statement, weird imagery and a teaching that no loving father would want to believe, that it is right to sacrifice your son - or anyone, for a principle.