the real question is why they cant get more market share.
www.idc.com/prodserv/smartphone-os-market-share.jsp
iphone 6 plus was a savior for apples market share. but where are all new customers? and apple has no competitor selling ios-devices.
why sales are fine? clever marketing and they can sell the same device twice for you. they do a phone now, it's a new, there is just some new thing(s) to call it a new, better, amazing phone. (6plus was actually something new) But, they dont put all things at the same time. they save something for the next model. A year later they have a new, amazing phone, which is basicly the same as the old one (e.g. 5 and 5s) but it is enough differend to call it as a new, amazing thing. if there isnt nothing, they dont hesitate to call a new air-thin volume button as a amazing volume button and people are sold. it is already a year ago when they bought the phone, they are ready to buy a new one! you dont need to do much, just tiny changes.
think about air and air2. what is the main difference? 2gig ram (and touchID) why didnt they put 2gig ram to air in the first place? because they needed a device which they can sell again a year later. 2gig ram was amazing new thing for air. then think about the difference between 5 and 5s. just a new processor and touchID. that was enough.
it is not hard to quess that in their next phone there is a 2gig ram. why they didnt put that to iphone6? they needed to save the ram for the next model to sell it again a year later when people are ready to buy again. trust me there is this new amazing 2gig ram which revolutionizes the ios experience with ios9.
it is a pretty clever marketing that they have managed to create:
you really dont expect to get much from apple, but just enough to buy it. when samsung for example announces a new phone, you are always expecting something real revolutionary or you are disappointed, because you have got used to it that samsung gives you always something new.