A large part of that is simply that smartphones entered the market very recently and started out computationally weak compared to existing computers - the first

A large part of that is simply that smartphones entered the market very recently and started out computationally weak compared to existing computers - the first iphone was released less than 14 years ago and had a CPU running at 412MHz! Smartphones have largely caught up to traditional computers in terms of the time it takes for upgrade-worthy advancements in hardware. I expect we'll see smartphone upgrade cycles continue to lengthen until they mirror desktop/laptop upgrade cycles.