It's just too damn hard to use for most kids. By the time they've worked their way around the Linux command line and file permissions, a big percentage have lost interest. (Yes, this is based on experience, from friends who are trying to use it in classrooms.)
It's good that the Pi Foundation is making an effort to produce educational materials. But IMO they should have worked much harder on initial usability, with a simple task-driven desktop that pointed kids straight at coding and web design.
With the old 8-bit micros you could start coding immediately. The Pi is nothing like that. The effort/reward function is off for all but the very brightest kids.
As for the Micro Bit - as you say, it looks a bit like the BBC's take on Arduino. I guess success is going to depend on the software environment, which is something that no one seems to have much information about yet.
I'm concerned that C++ is included, because that wouldn't be my first choice as a teaching language. But Python is too, and there's a custom language for beginners, so it may all work out.