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.