Double deckers fell out of fashion nationwide during the 90s as capacity wasn't needed due to falling patronage and using full size single deckers cut down on the vandalism and anti-social behaviour. Single deckers also had a better image with the travelling public.
I've heard some operators complain about limited garage space and so can't use full size single decks. So that means they can only use E200 size buses for single decks, meaning that they have been forced to go back to double deckers when a short single deck is too small.
Nottingham has bucked the national trend and increased patronage, so inevitably single decks have got busy.
One could argue that Trent started the trend (though in truth, these things are a bit cyclical). In fact, Midland Red got rid of virtually all their deckers during the 1970s with the exception of Leicester and got no new deckers from 1971-83!
The first new vehicles that the privatised Trent purchased were 24 Volvo Citybus deckers in 1988/9. Those were the last new deckers bought and since then, it's been various Optare products plus Darts, B10Ms etc.
Sometimes, it's not even companies but specific managers. The (in some quarters) respected Bob Hind who was of Arriva, didn't like DDs. Hence, Arriva North West didn't have new deckers for some while, yet as soon as he left for Arriva in Leicester, new and cascaded deckers poured into Merseyside!