Several theories have been put forward as to why this was. The best are:
1. Aventus had a unique voice actor, who was unavailable to do the supplemental dialogue necessary for adoption (might have had other commitments, or maybe his voice changed and he no longer sounded like a 10-year-old, who knows?)
2. The Aventus character itself is bugged, which is why he never returns to Honorhall. Attempting to patch the issue risked messing up the Dark Brotherhood questline, so Bethesda simply decided to leave him in his house. Out of sight...
I agree with you, it's too bad he wasn't adoptable. My Khajiit vampire assassin would have regarded him as the perfect apprentice.
Actually the quest is bugged and you can fix it with "bandit children" mod or any mod that makes children non essential.It makes Aventus non essential.He leaves his home 2 days after you complete the quest and does in fact return to Honorhall.I can post screen shots of him there.This is not intentionaly done by the mod.It some how fixes the bug where he never leaves and he actually walks back to Honorhall.
He is still not adoptable though,even at the Orphanage.
Angrenor the once Honored (beggar) takes up residence in tthe Arentino houshold exclaiming that he was very good freinds with the Arentino's and he is sure they would not mind him taking over there home now that they are gone.
weird right?I was like wtf is going on here????.When I first saw Aventus walk out the front door......lol
Oh.I have no idea why he is not adoptable.I would bet it's the voice.It is unique to him.
It's kinda cool though.When you walk into the orphanage ,if he noticers you he says "oh!! you came back to visit!!!"