It would've been more popular among that age group, and I just don't see older anime fans embracing a monster based anime series with crap like Pokemon and Yugioh tainting the genre. Plus if they wanted, they could have released a unedited version afterwards like they did with DBZ and One Piece.
As it stands now, it looks like this is going to simply drown on Adult Swim.
Revenge is a dish best served cold...
And, indeed, it did drown.
I keep seeing this on 'worst of anime on Adult Swim' lists along with Durarara!!, which aired at around the same time, and it's rather unfair to both series. Though, in the case of Kekkaishi, I think it would've done a lot better in a daytime slot. Sadly, CN seems to have no interest in giving anime other than Pokemon and other toyetic series a shot.
This is one of my favorite anime series.
It finished its arc in a satisfying way, even though many parts of the story were unsolved. The manga does complete the story, and I thought it quite clever and unique compared to the typical god-moding which ruins nearly every other series these days.