With real ales, people will go to specific pubs because they have guest drinks each week, spirits, well typically there are a lot, but often on single malts are stocked in multiple varieties. Spirits tend not to have a use by date as they are only going to evaporate, so the cost can be recovered over a year, two, perhaps more.
Soft drinks may only have a 6 or 12 month life cycle, if they aren't sold they go in the bin. Also, cans or bottles of soft drinks take up a lot of space in fridges, cellars and under bars, where a bottle of gin, takes the space of less than three cans of coke. Coke arrives in cases of 24 cans.
I don't know with a captive audience if pubs will change, if we're there on our own and they only have water or diet coke we can walkout, if we're with friends, and one person doesn't really like what they have are the other 5 going to walk out as well?
Traditionally, pubs have changed when it fits in with them, not before or due to customer demand, unless generated by the drinks manufacturers e.g. alcopops, J20 etc.
Not sure how we force change here...