Perhaps because (see brochure http://colour-laser-printers.co.uk/pdfs/CE913A.pdf ) this device is a host-based device, which means that it does not understand any of the traditional Page Description Languages (PCL5, PCL6, PostScript).
In the Laserjet Pro CP1020 Color Printer Series Software Technical Reference manual (http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c02542859-2.pdf ) is the statement (on page 38) that
- If you try to use any print driver other than the HP LaserJet Pro CP1020 Color Printer Series printer driver — for example a different HP printer driver, a Microsoft printer driver, HP Universal Print Driver, or a custom printer driver — the product will not print.