Fox has decided not to show anything regarding their upcoming films including Covenant for fear of piracy concerns.
https://www.scified.com/news/20th-century-fox-will-not-be-showing-an-alien-covenant-trailer-at-comic-con-this-year (https://www.scified.com/news/20th-century-fox-will-not-be-showing-an-alien-covenant-trailer-at-comic-con-this-year)
Fox has declined comment on the issue.
It's fairly old news, I'm afraid. I even mentioned it in this morning's news post.
http://www.avpgalaxy.net/forum/index.php?topic=54364.0