Juliet
It may be something to do with the criticality of the system from the certifcation point of view and it meeting all the relevant DO-178B, DO-254 and DO-160 quals.
On top of that it would also struggle to meet the FAR requirements for use of colour for warnings, cautions and advisories for the various flight information (speeds etc) if certified for use as the PFD.
This is a "off the top of my head " type of answer but maybe something to investigate further.