US is less on that nationalistic wave whether you take the positive or negative meaning of the word.
Smaller (relatively to giants like the USA or China) countries usually are more nationalistic and that means they will back the home fighter with a bigger chance.
I'm not sure but I also think the UK is a little less divided by race conflicts than the US.
plus: Brits are also big sport fans, they have the money to buy tickets, they have a live scene and a quite professional promotion for it. All these lucky factors results that they are the live gate heaven of boxing right now. Viewership, therefor money wise USA still reigns but as far as live gates I'd bet the UK is at the top.
in the USA people's concentration is also very divided cause there are many popular sports. In Britain football reigns supreme but other than that...

I guess cricket is poplar among Pakistanis and Indians but that's all.
not sure though...
@NoMyNameIs... can tell this better I guess