Soft drinks began as elixers and potions in the 19th century. Think about peddlers in the later 1800s selling their "snake oils" and potions. They were a sort of pseudoscience-based industry. You've probably heard that Coca-Cola used to contain cocaine. It was a way to give people a bit of pep. Likewise, that's what caffeine is in these subsequent iterations. As the pharmaceutical industry became more scientific, the soft drink industry evolved to be more clearly about recreational beverages (still non-alcoholic, hence soft drink as opposed to hard drink). You should also look into prohibitionists' push for people to drink non-alcoholic beverages around the turn of the 20th century. Around this same time, real scientists figured out how to synthesize caffeine, the compound they found in coffee about 100 years earlier. People really enjoyed the caffeine in their drinks because it gives them bursts of heightened alertness. In combination with sugar (sucrose or fructose), a lot of energy and pleasure sensation. You might also want to explore the addictive effects of caffeine and sugar. Then you can probably draw a conclusion as to why an enormous industry was built around these drinks.