It’s funny that the first paragraph of the letter says they believed it was best for Elon to join the board. Then the second paragraph says that Elon declined to join the board and they believe it was also for the best.
Elon I would imagine is way too busy with his other concerns to be distracted by a social media company, even though at the moment it does seem to carry an oversized amount of influence.
And Elon's brand is probably too randomly 'firecracker' for a company that's probably already attracting scrutiny from governments across the world due to the above mentioned outsized influence it has on discourse and opinion.