Telco marketing is pretty sophisticated these days, when Apple entered the smartphone market they went to the telcos first to see how their business model worked and how they could improve it. The "subsidized phone" distribution model worked well for Apple and Telstra are very good at leveraging their customer base. Telstra marketing is built around getting everyone on Apple devices on top tier plans to maximize their profit per user. There is not a lot of customer loyalty in landlines and internet anymore, they are commodity services, the iPhone is seen as being the jewel in the customer user experience. Telstra try to push customers into bundling services and offering discounts with the new iPhone as a key to maintain customer loyalty.
Having every iPhone is almost identical also allows the dumb bums to upgrade without too much pain, it makes doing support also a lot cheaper as the interface stays the same. With android the interface changes with every phone manufacturer, which is bad for telcos as every new feature requires training on the device.