For the same reason that if you put a hunk of chocolate in the oven and called it "hot chocolate," people would be confused that it's not a warm beverage of chocolate and milk.
That is, the phrase "virtual machine" is usually assumed to be the name for a piece of software that pretends to be some particular hardware. It is less commonly used to mean a "virtual machine", that is, not a noun unto itself, but the adjective virtual followed by the noun machine.
The term "virtual machine" is already pretty overloaded. This isn't referring to virtualized hardware in the VMWare sense or a language/platform virtual machine in the JVM sense. Rather, it's talking about how C's abstraction of the hardware has the Von Neumann bottleneck baked into it, so it clashes with fundamentally different architectures like the Burroughs 5000's.
That is, the phrase "virtual machine" is usually assumed to be the name for a piece of software that pretends to be some particular hardware. It is less commonly used to mean a "virtual machine", that is, not a noun unto itself, but the adjective virtual followed by the noun machine.