Interesting that if you count iOS, Android, Windows Phone and BB OS 10 as the top four mobile OSes, they run four different kernels, BSD/XNU/Darwin, Linux, WinNT and QNX. The four new competitors all seem to be using Linux (five if you count WebOS).
I suspect that's a cost-saving measure as much as anything else. If you're a scrappy small project, reusing the existing Linux kernel is probably a good move.
And if you're an entrenched BigCo you carry a lot of NIH belief along ;)
Actually it is more leveraging your existing experience. I don't know about Blackberry, but the common kernel between your desktop OS and the mobile OS is a good thing.
One of the drivers in making the purchase of qnx for RIM apparently was the integration play in the embedded space. Automotive for example.
Have seen not much fruit of that other than the QNX car project [0]. I remember seeing a Porshce at CES a year or two ago that had the system in it and it integrated with the Blackberry Playbook pretty well.