Just my humble opinion....
On the Business Pax<-->Leisure Pax spectrum, I've always thought of UA leaning more towards the Biz side while CO leaned more towards the Leisure side.
To me, that implies a couple of things:

• UA has more of a no-show and/or last minute "book off" a flight as the Biz travelers adjust their schedules. The Leisure traveler is more likely to stick to their "advance purchase" ticket as originally scheduled.
• UA has more last minute, high fare sales. They would gladly pay a $400 VDB to sell a $500+ seat.

So I think UA has had more of a need to aggressively overbook than CO.
Like other things, CO may think their method is best because it always worked for them. But being the world's largest airline changes the game. Count me as one of those that think the VDB program may start with the CO metod but slowly change back to the UA way of doing things.

And I do think that more hubs means they can raise the stakes (overbook more aggressively). You can't just issue a VDB and be done with it, they airline needs to find alternate transportation. More hubs = more choices, and higher chance that the choice will be on your airline.

Steve