Marketing companies have been focusing on the number of cores a smartphone has as a major selling point in recent years. The obsession with mobile processors has been a boon to the chip makers, giving added visibility to companies such as Qualcomm and Nvidia. Recently however, the chip industry has gotten to the point where more cores will result in diminishing returns.
It is generally accepted that four cores will be the de facto limit for now. More cores means less battery life. Chip making companies are now taking measures to maximize the battery life because that is what consumers have been shown to truly care about.