  We propose a scenario that explains the apparent nitrogen deficiency in
comets in a way consistent with the fact that the surfaces of Pluto and Triton
are dominated by nitrogen-rich ice. We use a statistical thermodynamic model to
investigate the composition of the successive multiple guest clathrates that
may have formed during the cooling of the primordial nebula from the most
abundant volatiles present in the gas phase. These clathrates agglomerated with
the other ices (pure condensates or stoichiometric hydrates) and formed the
building blocks of comets. We report that molecular nitrogen is a poor
clathrate former, when we consider a plausible gas phase composition of the
primordial nebula. This implies that its trapping into cometesimals requires a
low disk temperature ($\sim$20 K) in order to allow the formation of its pure
condensate. We find that it is possible to explain the lack of molecular
nitrogen in comets as a consequence of their postformation internal heating
engendered by the decay of short-lived radiogenic nuclides. This scenario is
found consistent with the presence of nitrogen-rich ice covers on Pluto and
Triton. Our model predicts that comets should present xenon-to-water and
krypton-to-water ratios close to solar xenon-to-oxygen and krypton-to-oxygen
ratios, respectively. In contrast, the argon-to-water ratio is predicted to be
depleted by a factor of $\sim$300 in comets compared to solar argon-to-oxygen,
as a consequence of poor trapping efficiency and radiogenic heating.
