They don't care

Persia was very far away from where Baha'u'llah lived so the Persian Bahais only got partial information. And they have mostly been oppressed since the beginning, so questioning their leadership was probably the last thing on their mind.

During the life of Baha'u'llah external affairs were in the hands of Abdul Baha, and internal affairs were in the hands of Mirza Muhammad Ali. This might explain why most Bahais in other parts of the world followed Abdul Baha, while most of Baha'u'llah's family and close companions followed Mirza Muhammad Ali.

Still, the fact that that Tablet was published in a book proves that people must have read it. I wonder why nobody took the statement to its logical conclusion.

As for the UHJ, I assume they are more than aware of the Faith's dirty secrets. They have an entire archive of Bahai writings, which was created after they asked all the believers to hand in their handwritten copies of Bahai writings, and which they refuse to share with anyone else.

https://www.reddit.com/r/FreeSpeechBahai/comments/1cq86pl/ambiguity_of_the_matters_of_successorship_in/