Bad choices on the part of the implementors?
Seriously, according to the standard, “Plain ints have the
natural size suggested by the architecture of the execution
environment”, which does mean a 64 bit int on a 64 bit
machine. One could easily argue that anything else is
non-conformant. But in practice, the issues are more complex:
switching from 32 bit int to 64 bit int would not allow
most programs to handle large data sets or whatever (unlike the
switch from 16 bits to 32); most programs are probably
constrained by other considerations. And it would increase the
size of the data sets, and thus reduce locality and slow the
program down.
Finally (and probably most importantly), if int were 64 bits,
short would have to be either 16 bits or
32 bits, and you’ld have no way of specifying the other (except
with the typedefs in <stdint.h>, and the intent is that these
should only be used in very exceptional circumstances).
I suspect that this was the major motivation.