Porque a AMD estava a primeira a lançar CPUs x86 (x86-64) de 64 bits.
the AMD64 architecture was positioned by AMD from the beginning as an evolutionary way to add 64-bit computing capabilities to the existing x86 architecture, as opposed to Intel's approach of creating an entirely new 64-bit architecture with IA-64.
The first AMD64-based processor, the Opteron, was released in April 2003.
Na verdade, no kernel, o suporte de 64 bits é chamado de 'x86_64' para se referir ao fato de que tanto a AMD quanto a Intel (e outras) implementam essas instruções.