Are all criminal trials open to the public? Why? or Why not?

Answer 1

Generally yes, although either side may request a closed proceeding to protect the identity of witnesses or for other security reasons. The court may order the press to be excluded and to close the courtroom to the public in the interest of justice and to ensure a fair trial.

Answer 2

Most are -- there are a few exceptions where specific security concerns require the courtroom to be closed.

And it's a constitutional requirement -- 6th Amendment : "In all criminal prosecutions, the accused shall enjoy the right to a speedy and public trial..."