For the same reason that diffie-hellman-group18 goes below group16:
it's useful to _have_ it there, in case a server demands it, but under
normal circumstances it seems like overkill and a waste of CPU.
SHA-256 is not only intrinsically faster, it's also more likely to be
hardware-accelerated, so PuTTY's preference is to use that if possible
and SHA-512 only if necessary.
(cherry picked from commit 289d123fb8bb8175c5a0d7a9944d45556f3886db)
aa87c20716