On a traditional motorcycle (or bicycle, for that matter) the rear wheel pushes the front. A two wheel drive bike allows the front wheel to pull also, which if you have the front wheel turned significantly and then power is applied to it, i.e. under acceleration, it tends to make the whole bike try to make itself stand upright unexpectedly and throw you to the outside. This has the net effect of widening your turn, which at speed is one of those things that's likely to result in you making friends with a tree. Or oncoming traffic, depending which way you were turning relative to the road.

You can already wash out the front wheel on a bike by braking, but on a two wheel drive machine there's also the new and exciting possibility of washing out the front under acceleration, as well. Losing traction on the rear while the front is still pulling in slippery conditions, i.e. those where a two wheel drive machine is most likely to be used, winds up with both wheels ultimately pulling in different directions, especially if the rear hooks up again unexpectedly. This will probably result in an unrecoverable tip-over unless your bike is very light or you're going very slowly. This may go some way towards explaining why that Rokon bike linked above has a top speed of 35 MPH.

You can sidestep all of this malarkey by getting a sidecar equipped bike with a driven wheel on the sidecar like the various Ural models. These have a freewheeling front like a traditional motorcycle, but a solid axle in the rear that drives both the bike's wheel and the sidecar's wheel for snow traction. It's certainly not impossible to tip over a sidecar bike (especially on the side that hasn't got the sidecar attached to it) but it's certainly a damn sight harder than a bike without one.