He knew what was likely his fate in Roman hands,
why submit after such a harsh struggle, not to mention doing it in such a personal manner with Caesar?
Is there anything suggesting that there was some kind of a deal to spare some Gauls from the wrath of the now battered and furious Roman army?
Update;
I believe I perhaps found the reason why in Caesars commentaries;
[7.90.3] I restored to the Aedui and the Arverni about 20,000 prisoners.
So the two main tribes got 20 000 of their people back, they evaded slavery, unlike the rest.
Perhaps this could have been a part of some kind of deal?