I've given my theory why I think Oath of Vengeance was left out, though I'd say that Oath of the ancients fits for a chaotic good character more than lawful good. As for why we pick oaths at first level now, I think it's becase there's nowhere to say the oaths now. Plus the specific oaths are tied to dialogue options so I think Larian wanted to provide more of those as soon as possible. Which I don'tthink is a bad choice necessarily. Also, if we don't have oaths from lvl one, then that's a decent stretch of time when we're not technically bound by anything and can act as we wish.
I don't mind the change, I don't think it alters the roleplaying or flavour of the class significantly.
