It allows companies to both sell into the existing ANT+ ecosystem and sell sensors which can connect to the iPhone natively through BLE.
ANT itself has design differences from BLE which means BLE cannot cover every use case that ANT can such as meshes or multiple transmitters to multiple receivers.
Plus from first glance ANT provides even lower power then BLE can provide for both the master or slave ends of the link. Theoretically I'd imagine a BLE slave would have a smaller stack then an ANT single channel device but Nordic themselves have admitted their BLE stack is almost 4 times larger then their ANT stack on their SoC.
BLE and ANT+ using the same frequency band. A mouse or keypad will always use BLE. For several sensor applications ANT+ is much better than BLE. Out of ANT and BLE are several further stacks. Anyhow, on the new nRF51 series you can run ANT and BLE. You can choose later which stack is better for your application.