Some analysts suggest that everyone who wants to own an Apple product already owns it and that Apple has already reached its peak sales.
The decline in iPhone sales was among the main reasons Apple's revenues were lower in 2016. Being Apple's primary product, any change in iPhone sales greatly affects the company. The iPhone accounts for two-thirds of Apple sales. See why some people love iPhones.
Some people believe that the iPhone 7 was not compelling enough to motivate a large number of people to upgrade their phones. The fact that the iPhone 7 is very similar to the 6s might have been a part of the problem. See why the iPhone 7 is not worth it.
Apple's Mac & iPad sales declined as well, thus contributing to the lower overall sales of the company. Mac sales declined 17 percent, while the iPad sales dropped 6 percent in the fourth quarter of 2016.
The economic slowdown in China has affected Apple's sales in the region. China is Apple's fastest growing market and so the sales decline there affected Apple's overall sales. Apple's sales in China declined 30% on year-to-year basis.
The strong dollar affected Apple's international sales as its products, which are priced in dollars, became more expensive in many of the countries that experienced a drop in their currency against the dollar. See why the Dollar rose in 2015 & 2016.
According to Tim Cook, Apple's CEO, Apple had some supply issues with the iPhone 7 plus as the demand outstripped supply.
Competition from other manufacturers, especially the ones in china such as Xiaomi and Huawei have also contributed to the sales decline of the iPhone.
The slower global economy might have motivated customers to save more and to spend on necessities. When the economy slows down, luxury products are usually the ones affected the most.