Upvote:1
It was a matter of medieval iconography, which in turn had a religious basis. That is, Christian themes such as God, Christ, and the Madonna, were depicted in standardized, stylized ways. Mere "kings" did not hope for any better.
Also, until the turn of the 15th and 16th centuries, most painters were also wooden "printers." Germany's Albrecht Durer was a case in point. So most painters, of portraits or otherwise, created paintings much like wooden print.
It was the Renaissance, with its emphasis on secular themes stemming from pre-Christian antiquity that broke the above mold. And the widepread adoption of Gutenberg's printing press caused the two arts to diverge, with people choosing one or the other. Concurrent events in art, such as the development of the "third" dimension, and the greater use of color helped artists to make that divergence.
Upvote:3
A personal opinion, which I admittedly cannot back with sources atm, but I suspect it had as much to do with the "rise of the individual" during the Renaissance. Writers, artists etc are largely, though not entirely, anonymous before that. The Reformation emphasised personal salvation, an individual relationship with the Divine, and the individual, as opposed to the group, community, caste or class, began to step out of the shadows - cf Hamlet "What a piece of work is a man!" Earlier, the individual was simply not important, so why would s/he be painted?