Most ticketing tools assume one event, one venue, one entry. A Hop On Hop Off tour is none of those.
Inventory is a route, not a seat map
Capacity depends on the vehicle, the loop and the time of day. A ticket is valid across many stops and for a whole day or more, so stock cannot be modelled as a single event.
Sales happen on the bus
Drivers and hosts sell on board, often with patchy connectivity. The system has to take payment and issue a valid ticket on the spot.
Validation at every stop
A single ticket is scanned many times. Generic tools treat a second scan as fraud; a route-aware platform treats it as a normal boarding.
When these are the core of the business, a purpose-built platform stops being a luxury.