You know, it might just be a matter of opportunity. I haven't done a reread recently, so I might be remembering some things wrong, but the Ten are dead and have been for a while. Yes, they might have done amazing things, but they've faded into obscurity in the sense that they really just don't come up in casual conversation all that often.
On the other hand, the Host are actively recruiting against a threat that has ruined quite a few lives. Given that Gramps seems to have tried his best to give his charges a good upbringing and relatively firm sense of morality, it's not that much of a stretch to think that they'd sign up with the Host and try and help out the people in front of them rather than join some obscure group dedicated to the Ten that couldn't do as much.