A further thought is that in the early days both bourbon and soft drinks were expensive. Only the well-off could have afforded either with any regularity. And being well-off, the people who knew these drinks would have had people with Jerry Thomas' skills to make cocktails (i.e., purpose-made) for them.
It was, arguably, only when both bourbon and Coke became relatively cheap that people, and perhaps younger people (at least nationally), thought to combine them.
But again I return to the question, which can be a subsidiary theme of this thread: does anyone drink, or has anyone heard a call for, bourbon and Pepsi?