Why? The Russians do not have an effective delivery mechanism for it.
The MOAB weighs in at ~9.5 tons... well beyond the carrying capacity of any kind of current missile system, so you need a conventional aircraft to drop it. Unfortunately due to the shear size and mechanics of such a weapon you can't just drop one from a
run of the mill bomber, instead you need something more like a C-130, a largely undefended cargo aircraft with a top speed that doesn't even hit 400 mph.
Translation... a sitting duck.
CNN said it fell from a Tu-160, much faster than a C-130 (1,380 MPH).