Problem is, if you use another OS, you have the problem of getting a decent app store going, and employing an Android runtime to get the decent ones - which is what Blackberry tried in BB10 and Jolla did with Sailfish. Both didn't go well - why bother with a phone that has a compatibility layer when you can get one that runs everything native?
Would be much less hassle, and increase greatly uptake if they went with something that runs Android apps natively.
If you can create a new mobile OS, and combine it with an app store that can rival that of iOS and Android, *and* convince devs to port their apps to it, you'd be in business. Which is what MS tried with Windows Phone and that didn't go all too well either.