BlackBerry vs Samsung vs iPhone security?

Fact: Out of 600, 000 phones used by the DOD, 470, 000 are BlackBerry's, 87, 000 are Samsung/Andriods, and 41, 000 are iPhone's.

BlackBerry has been chosen as the main phone while the others are BYOD (Bring Your Own Device) devices because it has passed their security screening. Seeing how BlackBerry has BlackBerry Balance and Samsung has Knox (both very similar) my question is how is the iPhone security? What makes the iPhone so secure from hackers?

Samsung