It meets a number of needs from what I can see:

Comfort paired with consistency (people recognise the name and know the flavour)… Addiction (caffeine hit)… Comfort again (the fizz can help with digestion if you have an upset stomach)… Sugar cravings (it looks like sugar is actually a highly addictive substance)… Safety (for example if you are traveling people will often only drink from a known safe container to avoid bad water)…