Of everything in a stadium, the plumbing is the hardest thing to change. Fixture counts are set during design, built into the structure, and inherited by every later renovation.
Codes set the minimum
Building codes specify fixture counts as a ratio to occupancy, broken down by type, and the design capacity drives the whole calculation.
Those minimums are exactly that. Meeting code produces a building that is legal and still queues heavily during a halftime that concentrates demand into fifteen minutes.
Assembly occupancies are the hardest case, because usage is not spread through the day but arrives in two or three intense waves.
Why adding fixtures later is so difficult
Restrooms need drainage running to a sewer connection, and drains rely on gravity, which means slope, which means space in the structure below.
Retrofitting a new group of fixtures into a concourse requires cutting the deck, routing new waste lines and finding capacity in the existing stacks.
All of that competes with structure, and the cost per added fixture in an existing building is many times the cost during original construction.
The distribution matters as much as the total
A stadium with adequate fixture numbers can still queue badly if they are clustered near some sections and sparse near others.
Spectators use the nearest facilities, so demand follows the seating layout rather than distributing evenly around the concourse.
Upper decks are usually the worst served, since the space available for back of house shrinks as the bowl narrows toward the top.
Allocation has changed and the pipes have not
Many older venues were built with fixture splits that assumed a very different crowd composition, and the resulting imbalance is now widely recognized.
Converting facilities is possible because it does not require new drainage runs, only refitting existing rooms.
All-gender and family facilities are added the same way, reusing existing plumbing locations, which is why they appear where the pipes already were.
The queue is managed operationally instead
Since fixtures cannot be added, venues manage demand by staggering breaks, adding temporary units at large events and directing crowds to underused areas.
Concourse design helps or hurts, because a queue forming across a circulation route blocks people who are not in it.
Where a stadium's restroom experience is genuinely good, it is almost always because the count was set generously above code at design stage, decades earlier.

