Rationale (August, 2011):

This is a C compatibility issue and has ABI implications; there was no consensus to pursue a change.

The definition of intmax_t and uintmax_t, inherited from C99, leaves open the possibility that the underlying types might not be the ones with the highest integer conversion rank. The requirements for these types deal only with the representation, not the conversion rank, and it is possible for, e.g., long and long long to have the same representation, although they have different conversion ranks. On such an architecture, chosing long instead of long long for intmax_t would be conforming.