I would argue your foundation is incorrect – that there aren’t “so many” using Blackberry. But let’s just say that is true…

There isn’t much within the US (or any) government acquisition rules that have to do with being of the best quality or security. It is almost always about the ability to meet minimum standards at the lowest cost (Challenger is just the most notable example). When it comes to technology, you have the added burden of inertia (Blackberry was among the first smartphones with a moderate level of security – not that they are even in the running in today’s world), and that many of the government people making decisions either don’t have a clue what they are doing, or were trained a decade or more back, and haven’t upgraded their skillsets since.