Just because butterbeer tastes good it's a better fit? There are plenty of reasons why HP is a great fit for theme park lands, but this is a pretty odd one.
Yes, Butterbeer is mentioned in the books, but if Universal didn't have good chef's to create a tasty specialty drink, then it wouldn't matter. Word of mouth would have been crap if the drink tasted bad. But it tastes great, so when people post on FB, Instagram or Twitter, or when they go home to their friends, they have nothing but great things to say about how good Butterbeer was.