Because they know they have a captive audience.

I worked in a pub for several years (way back in my dim distant past) and it's a fact that those drinking soft drinks, drink less than those drinking alcohol. If you have someone buying 3 pints of beer, you make more money than the someone buying 1 pint of soft drink,usually over the same amount of time. The more alcohol you consume, the more you want... the more cola or OJ you drink, the more you just need to pee.

If they thought there'd be money to be made in soft drinks, they'd open up a dry pub.

LTS