Was it because ELF is superior?  Better tools support?  Something
   else?


um... sorta yeah. :)


elf is less broken than a.out in many ways (in particular, it is quite
extensible locally for all sorts of purposes) but i don't think it is
the be-all and end-all of executable formats.  however, your second
guess is possibly the largest factor..  at least, IMO..  the fact that
ELF is supported and used by lots of other people using the GNU 
toolchain means that they maintain it for us :)  we get that "free"..
not just UCB-free or GPL-free :), but saving us large amounts of time..
eg, when i created the sparc64 netbsd compiler description, i pulled
on all the already-there support.  i had to create (via lots of copy
and paste :-) a header file, but didn't have to write any code.. and
christos made the NetBSD/sparc elf description in probably the same
fashion...  if i had to start from scratch, i'd have _never_ done it.


speaking of sparc ELF, time to updating more systems :)