Why photocopier is not called Xerox machine?
"Xerox" is a registered trademark of the Xerox Corporation, so
the phrase "Xerox machine" literlally means any machine that has
been trade marked and sold by or for the Xerox Corporation.
The word "photocopier" is a term meaning any device that uses
light to make a copy, and usually means a machine that uses dry
ink, plain paper, and provides its copies quickly after the subject
it exposed.
Since Xerox Corporation sells many machines that do not make
copies, and many companies, other than Xerox Corporation, make and
sell machines that make photocopies, the two terms can not be used
interchangably.