Many reasons. The closures started back in the 70's. There were almost certainly too many poorly run pubs and they died a death with customers going elsewhere.
Peoples drinking habits changed with supermarkets selling cheap booze, the after work drink and lunchtime office drinking culture changed about 20 years back. Ever increasing taxation drove the cost of a pint ever upwards - way above inflation rates. High business rates. The smoking ban alienated many and they switched to drinking at home, the drink drive laws put many off, the continuing hectoring of the anti drink health lobby and the ritual of reducing the "safe" level of alcohol every few years.
The turning of pubs into food based establishments ousted traditional drinkers. The advent of TV streaming and services like NetFlix encourage a stay at home culture.
The rising value of property has enabled pub companies to make huge profits from selling pubs off to builders and developers with no regard for the local customers. There have been countless numbers of the "old" traditional large pubs sold for housing in the last few years. see posts above for pubs like Hollies, Horse and Jockey, Sea View, Strouden Park, Ensbury Park, Portman etc etc
some of us do our best but i can't keep them all going !!!!!!!