The prevalence of clay courts tends to coincide with the warmth and humidity of the climate. Clay is much cooler under a hot sun than a hard court is. In some hot, dry climates, the difficulty of keeping clay courts watered makes them rare, but where heat and water are both plentiful, such as in the southeastern US, clay is especially popular.
https://www.liveabout.com/clay-courts-the-good-the-bad-and-the-debatable-3208212