Because AMD was the first one to release 64-bit x86 (x86-64) CPUs.
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.
In fact, in the kernel the 64-bit support is called 'x86_64' to refer to the fact that both AMD and Intel (and others) implement those instructions.
