You are not guaranteed to receive a rare item after opening a Black Lion Chest. You have a chance to get a unique/rare item. Some people are luckier than others. The Halloween skins are tradeable, so if you aren’t able to get them in a Black Lion Chest, you can obtain them on the Trading Post.
Thanks Regina, you are so cool! Maybe you can approximate the number value of this “chance”? You know, 1% is a “chance” and 0.01% is a “chance” too. But they are a bit different. Of course, it’s not a well-established practice among a developers to post exact rules for games they release, but maybe for Halloween event you will break this ill tradition and post exact number?