Other reasons...?
natural flavors, additives, etc.
Generally, soda is bottled by combining a few components and filling the bottles. 1. Flavor emulsion (known as the syrup or base) 2. Sweetener (usually hfcs) 3. CO2. The last 2 are known as Group 1 or generically kosher. Since it's cold fill, if the emulsion is known to be kosher, it's accepted as kosher. (This only applies if tbe emulsion is known to be kosher)
Teas on the other hand can be hot fill they can also have probiotics mixed in nowadays (kambucha) and with lesser known brands, it may not be known if indeed the emulsion is certified.