The North American stretch of the tennis calendar is played on hard courts almost without exception. The uniformity comes from what those surfaces cost to build and maintain rather than from tradition.
Hard courts are cheap to keep
A hard court is an acrylic surface laid over asphalt or concrete. Once built, it requires cleaning, occasional resurfacing and very little daily attention.
Clay requires watering, rolling and line maintenance every day, and grass requires specialist groundskeeping, growing seasons and rest periods between uses.
For a facility that hosts one tournament and serves club members the rest of the year, hard court is the only surface whose annual cost makes sense.
Climate rules out the alternatives
Much of North America experiences hot, humid summers and cold winters, which is difficult for grass and hard on clay courts that must be rebuilt after each freeze.
Grass in particular needs a narrow band of conditions and constant care, which is why the grass season worldwide is short and concentrated in a small number of places.
Hard courts tolerate temperature swings and dry quickly after rain, which matters for a tournament running to a fixed broadcast schedule.
Scheduling reinforces the uniformity
The summer events function as preparation for the season's American major, which is played on hard court, so matching the surface is the point of the series.
Players use the swing to arrive at the major with match practice on the same surface, which would be undermined if the lead-in events were played on clay.
Tournaments compete for entries, and offering the surface players need for the upcoming major is a straightforward way to attract a strong field.
The physical cost is real
Hard courts have less give than clay or grass, so impact loads through the ankles, knees, hips and back are higher over a long stretch of matches.
Players and coaches manage this by limiting the number of consecutive events entered and by adjusting training loads between tournaments.
Court speed varies between venues through the texture of the acrylic layer, so two hard courts can play very differently even though both are technically the same surface.
It shapes the American playing style
Growing up on hard courts encourages flatter hitting, earlier ball-striking and shorter points, because the surface rewards taking time away from an opponent.
Players raised on clay develop different habits, including heavier topspin, sliding movement and comfort in extended rallies, all of which are less rewarded on a fast hard court.
American development programs have deliberately added clay training to counter this, since the professional calendar requires competence on a surface the domestic system rarely provides.