The difference is that in the UK, you'll either have multiple flights a day or can take train/car without much delay instead. That's very different in more sparsely populated countries as the US.
But cash is likely to be the main issue here. If united had offered $800 cash, they would've probably found someone to rent a car and drive down there instead of flying.
Often this is true, yes. But I've also seen this happen with flights within Europe on occasion too - the bidding usually starts with £100-£200 cash plus a night in a good hotel with transfers dinner paid for, and a flight the next day. I don't think I've ever heard the auction go very much past that. I guess there is always someone not in a hurry willing to extend their trip by 1 day. Actually, I think I considered it myself one time, but someone else took the deal while I was still thinking on it!
Compensation is definitely the main issue. If United had offered a decent sum of cash (I mean cash, absolutely not United vouchers) they would certainly find someone willing.
But cash is likely to be the main issue here. If united had offered $800 cash, they would've probably found someone to rent a car and drive down there instead of flying.