There are many reasons:
1. The economy - people are feeling the pinch and aren't spending as much
2. Drink / drive - people won't drive to pubs and taxis are more expensive now
3. Cheap booze in supermarkets
4. Land prices - pubs are being pulled down to make way for housing
5. Trouble. Many 'decent' people don't want to go out drinking at night for fear of getting into fights and witnessing other bad behaviour.
But pubs were closing long before the smoking ban, as were breweries. M&B sold off their large brewery in Birmingham 5 years ago to a housing company; Davenports sold out to Carlsberg in 1992 and the Birmingham brewery was sold for housing develpment.
In 2005, 26 pubs a month were closing:
http://news.bbc.co.uk/1/hi/uk/4735503.stm