"intel architecture" is commonly used to refer to x86 and x86-64. x86 is normally used to refer to 32 bit architecture - hence D uses X86 for 32 bits, and X86_64 for 64 bits. Calling the file x86.d will be confusing.

Also consider that the names of the files can be changed at will without affecting user code at all. It's not terribly important what they are called.