As I understand it, GCC and gas for i386 were written to be compatible with existing Unix C compilers (emitting assembly) and assemblers (consuming assembly), a

As I understand it, GCC and gas for i386 were written to be compatible with existing Unix C compilers (emitting assembly) and assemblers (consuming assembly), and those tools used the "AT&T" syntax that copied the PDP-11 assembler's syntax. For amd64 there isn't really much of an excuse. Intel syntax was already supported in gas.