User blog comment:Dominic1743/“The Trap” Discussion/@comment-74.132.82.188-20200119010843/@comment-108.36.76.116-20200122101435

The reason it went to Sam and not Dean is that Dean already took the Mark once. It's not something you can do over. This came up when Amara was attacked by witches, angels demons, and Lucifer. Then, Chuck was going to give Sam the Mark. Dean protested and said he'd take it again. He was told he couldn't, because Dean had already been tainted by it once. Sam said he had to do it himself, and he and Chuck had already discussed it. Dean is ineligible to receive the mark, so it could have been Castiel or Sam, but since Castiel was holding it and cast the spell, it had to be Sam.