I get the feeling that USA is just as fragmented as anywhere else. Here (Mid-Atlantic USA) is 29er central! 90% of the bikes on the trails here are 29ers and 75% are hardtails. (The trails here are far more ‘technical’ than most stuff I rode in 7 Stanes/Lakes/Peaks/Dales in a different way. Trails here are tight but flowing, short sharp technical climbs, rooty in places, massively rocky in other places.) People don’t ride in the wet for reasons already dealt with.
However, people who have ridden in New England, or the West Coast say 26er full sus bikes are the choice there, and it sounds pretty similar to the UK, trails are ridden all year round. The USA is a big place…
The difference in rider style between Inners/GT (my old stamping round) and here is night and day. But I wouldn’t want to ride my UK bike on these trails any more than I’d want to take my SS down the north side of Nan Bield.