As understand it, the 4D string landscape is a function that assigns an energy to every possible compactification of the 6 small spatial dimensions. We expect our universe to lie in a local energy minimum, and if there is a lower minimum at another compactification, then our universe would only be metastable, because we expect that it would eventually quantum tunnel into the lower minimum.
What is the process that prefers compactifications with lower energy? I can understand why we might expect total energy should be conserved, but not why it should be minimized. To me, the logical statistical ensemble with which to describe the universe would be the microcanonical ensemble, in which the total energy is arbitrary and not necessarily minimized.
Usually there are two processes that cause a system to lower its energy: (1) a dissipative system (e.g. one with friction or air resistance) continuously loses energy into an external sink, or (2) a system in thermal equilibrium at zero temperature minimizes its energy because it is coupled to a much larger external heat bath which gains so much entropy from its energy that the total entropy is maximized when all the energy leaves the system and goes into the bath. In both cases, the idea is that the entropically favored situation is the one in which all the energy leaves the system and goes into some external "environment." But by definition, the universe cannot be coupled to any external environment, so there is nowhere outside of the universe for the energy to go. So why do we expect low-energy configurations to be any more likely than high-energy ones?