Originally Posted by Trakhak

Sticking with a full-size rear wheel meant that they could use, e.g., conventional drive train components, thus avoiding, among other problems, disproportionately large chainrings necessitating nonstandard front derailleurs. Most bikes with matching wheels that are smaller than the standard sized for adult bikes are geared low enough to be arguably inappropriate for anything other than short, casual rides. That was the thinking, anyway.

Originally Posted by JohnDThompson

Because a smaller rear wheel would have required non-standard size cogs and chainrings to retain reasonable gearing for adults.