Much has been made of the absence of President Donald Trump from John McCain's funeral, to which the sitting POTUS was not invited, as well as the absence of Former Alaska Governor Sarah Palin, even though she was McCain's vice presidential candidate in 2008. But no one has seemed to notice the absence of former POTUS Jimmy Carter from the highly attended event for the upper crust elite of United States politics.
Maybe it's for speaking the truth– which is never welcome or helpful to politicians.