1. Gee-whiz technology, with non-trivial downsides, that could be obviated with a third wheel. [1]
2. Everyone-will-want-one marketing for a device with deal-breaker-level problems that are trivial to identify. [2]
The Segway was pitched as solving a problem that was clear and known: bicycles are great, but they're only appropriate and convenient for a certain range of riders and situations. And, in short, the Segway just doesn't present a better solution for most of those situations or riders.
[1] You mention people taking assisted tours. But how much larger would even that market be, if it included people who could stand, but couldn't risk going over the handlebars while learning to ride?
[2] A device of most interest in relatively population-dense settings that not only has no solution to, but actually amplifies the problems of a bicycle? e.g. stairs, parking. (by being massively more expensive and heavier)