Because first they'd need to spend 6-9 months developing a current-ish version of Android to run on it (we're assuming you are't looking for a dev Lollipop build with known issues). And, then, once you had it, you'd inevitably want BB to *maintain* it.
BB has better things to do with their time and money than create a new OS build for a 3-year-old phone, especially since they've mostly left the smartphone business itself. What's in it for BB? Only more losses.