I saw this video on YouTube and was fascinated by it.
The bear and the person can move on the thin rope due to the large moment of inertia created by the huge mass of the bear and person.Assuming the mass of the person is 70 kg, the bike 180 kg and the bear, 200 kg with a height of 1m. Assume the person is 3m away from the tightrope. If they are both treated as point masses, the moment of inertia will be;
I = 70(3^2) + (100+180) (1^2) = 910 kgm^2
This is a large moment of inertia and would help keep the system in balance so it does not topple over.