Changes in the mobile market
I believe there are two changes in the mobile market that are in effect here.
Firstly, improvements in smart phones have slowed enormously. This means there are fewer "must have" features for people to have. Their current phone is good enough so why buy a new one?
Apple now offer 0% finance on their phones. One of my kids bought a new iPhone from Apple on the 0% finance and the Apple sales droid said that since they started offering 0% finance, sales have increased. With people being able to get 0% finance from Apple, shorter term/more flexible SIM only deals for airtime will be more appealing to people.