Less handsets per year
Apple produced, until recently, one size iPhone. It allowed all third-party accessory sellers to maximise their profits without having to design more sizes. That has slightly changed now with the new iPhone and even the change to 4" prior to the new phones. Still, that's only two phones that they have to build accessories for right now.
Look at other manufacturers such as HTC who, despite saying they'd reduce their product numbers, still pump out way too many phones in a year. If they stuck with the original One lineup (X, S, V) but with better specifications they would have a device for each price point. Instead they have the One M8, the One mini 2 (really great name..) and a bunch of Desire (9 at last count) phones at various specs for sale. Throwing a bunch of crap and hoping something sticks is not a way to succeed when you don't have a large budget. Samsung can afford to do it, HTC and many others cannot.