UUCP wasn't written until '78 which was much later than we're talking about, though the Internet transition to TCP (and thus, for this conversation, SMTP) didn't happen until (my memory is hazy) '82 or '83.

Also UUCP was for unix machines only (at least back then) which were a small proportion of machines on the net. Most of them were 36-bit machines plus a few IBM machines. You can still see the old 36-bit influence in some contemporary protocols (e.g. a high level protocol like FTP has a commend to set byte size), big-endian traffic etc.