IIUC, largely because of being surpassed by eastern europe.
https://public.tableau.com/app/profile/worldhappiness/viz/20...
EDIT: here's 2023:
https://public.tableau.com/app/profile/worldhappinessreport/...
{'czechia', 'slovenia', 'kuwait', 'united kingdom', 'costa rica', 'united arab emirates', 'lithuania', 'belgium'}
which I'd group as:
eastern europe CZ LT SI
oil middle east AE KW
western europe BE
latin america CR
airstrip one UK