I worked in government mobile security for many years, until last November. Many agencies are indeed rolling out iPhones and scaling back on blackberries finally
That said, Apple's enterprise business grew 40% in 2015 to $25B. (http://appleinsider.com/articles/15/10/27/apples-enterprise-...). .gov is a big part of that. When you attend Apple enterprise-focused events, the attendance has grown beyond the old media/field service crowd to include finance,police and .gov attendees.
If you need to comply with various federally mandated compliance regimes, (ie. FIPS 140-2, CJIS, IRS Pub 1075, etc) iOS is very clearly the easiest and most straightforward platform to achieve that compliance on. It's not as trivial as it was with legacy BlackBerry, but it's pretty close.
With Android, the carrier and manufacturer variance makes it more difficult to achieve, demonstrate and maintain compliance. That said, Android offers a number of advantages from an application perspective, but when in production, you'll commonly see licensed third party software in place to perform typical mail and other functions.
BlackBerry failed to keep the positive aspects of it's legacy platform fucntioning when modernizing the platform.
I used to run an environment with something like 25k devices... Provisioning and management was braindead simple with legacy BlackBerry, and it's integration model for mail & calendar was robust and reliable. Most users considered BB more reliable than any other mail access methodology. PIN messaging was device-centric vs. identity centric, which made it attractive or many use cases. That also went away, along with the perceived and real security benefits.
That benefit became a liability -- BlackBerry was very "enterprise telecom" and mail-centric. So the people who were responsible for BlackBerry were very much affected by tunnel vision. That's why BlackBerry was surprised by the market shifts -- their customers were very happy, but were ultimately people with an overly narrow focus who were disconnected from the business.
When BlackBerry switched to the ActiveSync model, they became just another ActiveSync device, with the added "benefit" that the rest of the platform was a mess. They are now trying to leverage their past reputation for good management to become an MDM vendor for some reason.
iPhone slaughtered Blackberry by attacking their strengths... BBM/PIN messaging with iMessage (with the added feature that you cannot intercept the messages if the feature is enabled), the obvious improvements of iOS vs. Blackberry OS and an easy institutional management model.
Poor sales, major benefit of blackberry overshadowed by other services (BBM), late to adjust to the times, etc.
I doubt you were suggesting that BB's downfall is rooted in their encryption, however. In the slim case that it did, they didn't speak up about backdoors and letting .gov into their devices until 2015, way after their market share has shrunk.
While I havent seen heavy market in that aspect there appears to be some level of improvisation with Blackberry's messenger http://www.bbm.com/bbm/en.html
