Why is the Nintendo Entertainment System (NES) referred to as an 8-bit system, rather than a 1-byte system?

As far as I’ve understood it, referring to this system as an 8-bit system points out that one can access 8 bits of data in one instruction.

While I understand that we’re not saving vast amounts of time by calling it “one byte” instead of “eight bits”, is there a particular reason why the latter is/was preferred?