The lack of a thermal ALD process for InN, which is known for AlN and GaN, indicates a reaction path for adsorption of intact NH3 on AlN and GaN, that is not accessible on InN. In this study we use quantum chemical modeling of NH3 adsorption on methylterminated GaN and InN. By comparison of adsorption energies and barriers we find that there exists a possible adsorption path of NH3 on GaN, while the similar path on InN would be orders of magnitude slower. We argue that this huge difference in reaction time would explain why adsorption of NH3 is severely limited on InN and thus that a fully thermal, low temperature ALD process of InN with TMI and NH3 would be very challenging.