That's a fair point, and I agree we do not know what God's intention is. But I would take it that if God sends messenger's the purpose of that is to send a message for mankind to know.
That is the purpose, but that does not mean everyone is going to recognize the messenger and know the message within any given time frame.
I would say that if no human is to blame for it, then God is. God is the one who performed the action (sending the messenger), and if most people don't hear that message or accept it who is to blame except God? Atheists cannot be blamed, because they simply do not believe the man. Baha'is cannot be blamed, because they are actively spreading the message. The messenger is not at fault, because he did his part. I would say the fault lies in the means (sending the messenger).
To say that God is to blame is completely illogical because God is infallible so God cannot make any mistakes. Moreover, God is All-Knowing and All-Wise so God knows the best way to deliver a message to humans. By contrast, humans are fallible so humans can and do make mistakes. So if Baha'u'llah was a Messenger of God and atheists, or those of other religions, do not recognize Him as such then they have made a mistake. We all make mistakes since nobody is perfect.
The fault cannot lie in the means (sending the messenger) since God cannot be at fault because God is inerrant. The expectation that more people would have recognized Baha'u'llah by now is a false expectation since there is no reason to think they would, given all the impediments in their way.
Below are the seven reasons why more people have not recognized Baha’u’llah yet.
None of them have anything to do with God or Baha'u'llah. All of them are related to human behavior.
1. Many people have never heard of Baha’u’llah, so they do not know there is something to look for. It is the responsibility of the Baha’is to get the message out, so if that is not happening, the Baha’is are to blame. However, once the message has been delivered the Baha’is are not to blame if people reject the message.
2. But even after people know about Baha’u’llah, most people are not even willing to look the evidence in order to determine if He was a Messenger of God or not.
3. Even if they are willing to look at the evidence, there is a lot of prejudice before even getting out the door to look at the evidence.
4. 84% of people in the world already have a religion and they are happy with their religion so they have no interest in a “new religion” or a new Messenger of God.
5. The rest of the world’s population is agnostics or atheists or believers who are prejudiced against all religion.
6. Agnostics or atheists and atheists and believers who have no religion either do not believe that God communicates via Messengers or they find fault with the Messenger, Baha’u’llah.
7. Baha’u’llah brought new teachings and laws that are very different from the older religions so many people are suspicious of those teachings and/or don’t like the laws because some laws require them to give things up that they like doing.
Have you heard of a little game called "Telephone"? I used to play that when I was young, sitting around a campfire with a circle of people. One person would whisper something into the 2nd person's ear, and the 2nd person would whisper it into the 3rd person's, etc. Along the way this message could get distorted for various reasons: Someone misheard the other, someone misspoken, someone along the way just wanted to say something completely different to mess with everyone...
My point is, this is how it always goes when you deliver messages through a middle man. When someone is not direct about what they want to say, even God, then there is bound to be misunderstanding or people who do not believe the middle man.
God was direct in what He wanted to say to Baha'u'llah so there need not be any misunderstandings regarding the messages that came from God through Baha'u'llah. Moreover, the Tablets were written in His own pen as the revelation was received from God, unlike any of the older religions whose scriptures were written by men many decades later, often by unnamed men.
Obviously there are going to be people who don't believe the messenger, that is unavoidable, but if God spoke to people directly there would be no way for anyone to know it was actually God speaking. Some people would not believe it was God and some people might think they were having auditory hallucinations.
In fact, the same issues that would come from sending the exact message to 7.9 billion individuals are faced when the Messenger sends the message to these 7.9 billion individuals, except with the added on issues of having a middle man send the messages.
If God spoke to everyone directly people would interpret what God said with different minds so they would not all understand the message the same way. It is no different with the messenger. People interpret what He wrote using their own minds so there are some slight differences in understanding what He wrote. However, there are no 'added issues' of using a middleman except that people have to recognize that middleman as a Messenger of God. That is the biggest issue that I see.
Why would they need to write it all down when they have the revelation? I suppose if there was a need to, there'd be more people to write these revelations. We could perhaps produce 1 billion tablets!
If they did not write it down, how would they remember it next week, next year, 10 years from now? There is a reason why Baha'u'llah wrote scriptures, since they will be needed for generations to come for at least the next 1,000 years, until another Messenger appears.