Gotta remember that the award is voted by sportswriters from around the country. These writers don't get to see every rookie extensively so probably base their votes on the little they have seen and raw stats. Jaylen hasn't posted any raw stats this year that will get him the attention needed to come close to winning the award.
Does that mean I want any other rookie this year over him? Does that mean he won't be one of the top rookies to come out of this class? Does that mean he doesn't have star potential? No to all of the above. It just means Brown was developed slowly here, didn't get the minutes that rookies on bad teams got, and so didn't show enough or post enough stats to come close to winning the ROY award.