Taxes, yeah right. Many of the shops only opened in the first place to skirt the four machine per shop law.
I don’t really pay much attention to TV adverts but when was the last time any of them encouraged you to go into a shop? Do the shops have the exact same offers and promos as online?
Unless you’ve been banned online (and even then shops can and do monitor and restrict you) or want to use cash why would you go into a shop? Yes there’s the social aspect for a few, although you can get the same social aspect in a pub punting on your phone with the benefit of being able to have a pint.
Too much supply of betting shops and dwindling demand, simple as that.