Depicting people one had essentially two methods: Making a three dimensional replica, or mapping what we believe to see on to a surface. These days, with glowing rectangles pretty much 99% of where our attention goes, most of what we see if the later kind.
Historically it was not the case. Comparing both methods in antique times it is obvious that sculpture is way more skilled and truthful that wood panels or frescos of the time:
400 years difference, but everything else that I could find as an example for a flat depiction was so bad that I felt it would over stress my point.
Once the dark ages were behind us it seemed that painting had caught up:
Again: there are countless painters that would have made my point better than van Honthorst. But even if with him we can clearly see that painting catches up.
Why? Or why did the skills of antique artisans did not flow as unencumbered into flat works?
One could conjecture that glas coverings for windows had a significant influence. During antique times wood panels and frescos where the main means of depictions. A fresco you can hardly sell or steal. Wood panels back then were small, and would ago not very well. Only when canvas became available a large flat pane became available to painters. These would also scale up very nicely. And a canvas was an instant object. Heck: you could even roll it up for easier transport and storage.
Humidity control is an important factor in the conservation of canvas and wood panels alike. A hygrothermograph in the corner of a room is usually a good indication that you might see something interesting and or of value on it walls.
Before there were glas panes covering windows it would also mean that you would let weather in together with sunlight. If it should snow in Rome then you see flakes descending the dome of the Partenon.
Having a closed room with a fire in it is a crude but possible solution, but you lack the light that you would need to appreciate a painting. So, only once people have rooms that were bright(ish) and dry(ish) would it make sense to invest allot of efforts to make movable pretty things that you put on your walls.