Cranial modification is actually a widespread practice. In addition to North and Central America, it can be seen in South America, Europe, Asia, and the Pacific Islands, in some places even into modern times. Its spatial and temporal ubiquity mean it doesn't imply any cultural connection. As for the why, it varies depending on the culture. In Mesoamerica, it appears to be both a status and ethnicity symbol, with the best techniques reserved for the aristocracy, in emulation of various Maya or Olmec gods. This is in contrast to places like the Andes or Eastern North America, where the practice is seen across all social levels. Here, different styles of modification are often thought of as irremovable ethnic identifiers.

If the topic interests you, I'd recommend finding The bioarchaeology of the human head : decapitation, decoration, and deformation at your library, as well as Vera Tiesler's The bioarchaeology of artificial cranial modifications.