Gamebryo is a general purpose 3D game engine like UE3. The Oblivion engine, which uses Gamebryo, is what I think you're getting at. It's tried and tested... for making continuous open world games like TES and Fallout 3. I hope this is an entirely different type of experience. That would seem like the best option for a spin off like this, otherwise it's just a mediocre sequel-oid.
Onyx was built from the ground up by Obsidian to make RPGs. Couldn't this make a more interesting game than the same Fallout 3 in a different setting?
Very much so, I love using new engines. I wasn't too sure what to call the engine, as many games use different engines but I usually refer to them as their original title. Such as the Unreal 3 engine for Gears of War and Aurora for The Witcher. How long did Bethesda release Oblivion for PS3 after the PC and XBox360 version, it took that long to make the game work for it. If Onyx isn't already designed to work for the PC, XBox360 and PS3 that is a big strike against it.
Obsidian have a deadline, given to themselves or by Bethesda (I guess the latter). Since we KNOW the Gamebryo/Fallout 3 engine works on the PC, XBox360 and PS3 getting this game to work using a new engine may take up their valuable time. Also remember since Bethesda are involved, wouldn't they want Obsidian to use their engine, to keep it similar to their exisiting game? Also since many elements are already available in the Fallout 3 game I'd say that that Obsidian can use those. Such as models, textures and game rules. You may not like Bethesda's versions, but they did sell well and money must be the biggest influence on this decision.
My posts are speculation based on the limited information I have, it could be that I am completely wrong but without additional information I can't see that I am. This is a good source for info at the moment: http://www.shacknews.com/featuredarticle.x?id=1113