A tour that misses an obvious city is usually read as a judgement on the audience there. The decision is more often made by the routing, the venue inventory and the cost of a day without a show.
Idle days are the expensive ones
A touring production carries a payroll of musicians, crew, drivers and technicians that continues whether or not a show happens that evening.
Trucks, buses and equipment hire run on the same continuous basis, so a day off costs nearly as much as a performance day and earns nothing.
Routing therefore aims to link cities that are a sensible drive apart, and a city that would force a long empty run between two dates gets dropped even where demand exists.
Venue availability decides the calendar
Large venues are booked far ahead and shared with sport, conferences and other touring productions, so the free dates in any city are limited.
A tour is built around the dates that happen to be available rather than around an ideal sequence, and one unavailable weekend can remove a city entirely.
Smaller markets are hit hardest by this, because they have fewer suitable rooms and a single conflict leaves no alternative.
Production size limits where a show fits
Modern arena productions carry rigging loads, power requirements and stage dimensions that many older venues cannot physically accommodate.
Scaling a show down for one date is possible but expensive, since it means a different rig, different truck packing and additional rehearsal for the crew.
Most productions choose to skip the venue instead, which is why a tour can play a smaller city with a modern arena while missing a larger one without.
Load-in and load-out set the pace
A large production takes most of a day to build and several hours to dismantle, and the crew has to travel overnight to repeat it.
Some tours run duplicate sets of staging that leapfrog ahead, which allows consecutive nights in distant cities at a substantial extra cost.
Without that duplication, the geography of a route is constrained by how far a truck can travel between a late load-out and an early load-in.
Guarantees change the risk balance
Promoters commonly pay a fixed guarantee against a share of the takings, so the local promoter carries the risk of an underfilled room.
In markets where no promoter will offer a workable guarantee, the date does not happen regardless of how the artist feels about the city.
This is also why routings shift between tours: a market that performed poorly once becomes harder to book next time, and the absence compounds.