Aiken had never played football until last season. To understand why Bryan wasn't rated you have to understand how the recruiting rankings work.. They find these guys, usually by the time they are freshman, some times sophomores and they have a temporary rating system. They see what offers they bring in, they see how they play their junior season, they see what camps they attend, etc. and they give them a ranking. It is a good system, and a ton of work goes into it, but it isn't fool proof.
If a guy like Bryan doesn't play football until he is a senior he is already past the point where any of the services will care to go back and rank him even if they find him, which Rivals did. They almost wrote a story on him when they went to see Reggie at Rossview.
Even the P5 colleges already have their recruiting classes in order by the time these guys are seniors. Think about this time last year and the head start the Tennessee, Alabama, Georiga, etc. coaches had on the 2019 class already in terms of offers sent, guys scouted, whatever, and then think about the fact that Aiken was not even playing football until months later. That's the only reason Bryan Aiken wasn't ranked. If he had played all four years he would have been a five star, top 20 player in the country. I have zero doubts about that.