Why does ligand substitution occur when OH- is a better ligand than NH3?
This is an equilibrium reaction. Using LeChatelier's principle, you can figure you that if you increase the concentration of NH3 sufficiently, you shift the equilibrium so far to the right that you get the hexamine as the main product. At neutral pH in water, the concentration of hydroxide is quite low (100 nanomolar). You would expect this reaction to be pH-dependent, and of course depend on the concentration of ammonia in solution.