You are making an assumption that God cares how many people get the message. Why would that matter to God how many people get the message or how soon that happens? God does not need anything for Himself because God is fully self-sufficient and self-sustaining. The message is for humans, so they are the only ones who suffer if they fail to get the message.
That is because you have a particular goal in mind. My boss also sends e-mails to the whole section to be sure we all get the same message, if the message pertains to everyone and he wants us all to get it.
Apparently, you are assuming God's goal is that everyone gets the message that is delivered through the Messenger. Apparently you are assuming God wants everyone to know that the message came from Him. If God wanted all of us to know it came from Him, God could have made sure we all know that, since God us omnipotent and omniscient.
Obviously, if God had that goal in mind, God would deliver the message some other way, not through a Messenger who most people are not going to recognize.