Why wouldn't you classify PostScript as a programming language?

From its page on Wikipedia:

"PostScript (PS) is a dynamically typed concatenative programming language"

Just because it's mainly been used as a page description language for printers doesn't mean that people haven't written applications in it - Arthur van Hoff even wrote a C to PostScript compiler in PostScript (pdb - allegedly standing for "Pure Dead Brilliant" - it was written in Glasgow).