Why does the Sierra Madre del Sur have little flat land?

The Sierra Madre del Sur contains very little flat land because it is a highly dissected landscape characterized by a labyrinth of narrow ridges and steep-flanked valleys.1

Unlike the Mexican Plateau, which retains a relatively uniform volcanic mantle, the Southern Highlands—where the Sierra Madre del Sur is located—have been stripped of much of their volcanic cover. This process has exposed older basement rocks, resulting in what is likely the most highly dissected terrain in Mexico. The region is defined by this rugged, broken topography rather than level plains.1