Why does the East Pacific Rise have a broad high instead of a rift valley?

The East Pacific Rise features a broad axial high rather than a rift valley primarily because of its rapid rate of seafloor spreading.1

In the study of oceanic ridges, the morphology of the crest is closely linked to the speed at which tectonic plates diverge. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are characterized by a deep, fault-controlled rift valley at their crests. In contrast, faster-spreading ridges lack these valleys.1

The East Pacific Rise spreads at rates reaching 159 mm (6.3 inches) per year or more in some areas, which classifies it as a fast-spreading ridge. At these high velocities, the continuous, robust supply of magma and the thermal state of the crust prevent the formation of a deep rift valley, resulting instead in a smooth, elevated axial high.12