That's perhaps because the US doesn't have a CAMRA and thus has a craft brewing industry that's willing to try new stuff...
eh? CAMRA came about to support small breweries from being overrun by the large corporates - fail to see the connection here...
CAMRA's definition of real ale is fundamentally conservative and retrograde with the result that (I've been told by numerous British expats in the craft sections at the local beer emporia) there isn't much difference between one real IPA and another. The US craft brewing industry has evolved to where the various craft IPAs on tap at a pub that has multiple choices in that area or at a off-license will have noticeably different flavor profiles, abv's, etc., thus making it more likely that one of them is just right for one's tastes (and also making more to be snobbish about... e.g. you can say "Dogfish Head overhops all their beers and I never drink them").
Saving the British craft brewing industry is laudable. The US missed its chance with Prohibition (only the megabreweries survived, and then with the consolidation after WWII only a few survived into the 80s), but the blessing of that episode was that when craft brewing re-emerged in the 80s (and then grew like crazy in the 90s before shaking itself out in the early 00s) it was largely unshackled from the past.
A strong case can be made that the USA (at least in the states that don't outlaw beer with more than 5% abv) is the best place in the world to be a beer lover, despite the prevalence of Bud, Miller, and Coors.
The site of a local beer emporium (and their current flyer)
I can see where you are coming from but I don't think CAMRA is restrictive in the way you think. For example they put pressure to bring in legislation allowing landlords in brewery owned houses to stock at least beer from another brewery as a guest beer. There were no conditions as to the nature of this beer, it didn't have to meet any CAMRA standards or anything it just had to be brewed by a different brewer. (This has since been repealed) Obviously their festivals will stock beer that meets their definition, the main feature of which is that secondary fermentation takes place in the container the beer is dispensed in. I also think it isn't true that one IPA tastes like another!! Not at all to me, some I love, some I hate!!! I think there is more chance that british microbrewers just choose to produce traditional beer! Which is great Not knocking the US producers at all and its great they are doing well, but for me the UK & Ireland, Germany Belgium and Czech Rep possibly are the big countries for diverse beer.