No doubt the fact that BlackBerry is NOT a US company - which you can’t even say about Nokia now - helped. But the main reason was encryption. BlackBerry supports encryption acceptable to German security. Out in the cold is Apple’s iPhone which doesn’t.

Chancellor Merkel’s upgrade came after Germany’s parliament voted itself new guidelines to mandate encryption on all parliamentarians' phones following the revelations that the NSA was not only bugging the Chancellor’s phone, but that both the US and UK embassies were snooping on radio transmissions from the roofs of their buildings. Cue outrage, shouting and hurried diplomatic visiting in both directions. The dust may have settled now, but the encryption will stay on.