Because lore is now dead, and that ED lore was mainly via text to begin with. The leaks in beta builds might indicate more of these characters getting visuals.
Well the bobble-heads for them were seen in silhouette back in the day - they just never made it to the store.The other question and one of my personal wants is to convert the Powerplay leader portraits into 3D avatars (with themed clothes). The advantage then would be you could monetize it (Mahon slacks 500 Arx! Archon pirate coat 600 Arx!) and that avatars can be happy / sad / angry / pleased at your powers performance for that cycle.
Looking at that list, the first "important" character I recognize was Admiral O'Brian... who was "important" about 200 years ago, when the war criminal led Federation troops to seize the world Emerald, which the Empire had previously terraformed. I'm not sure why a character mentioned only in two places: A Frontier: Elite II gazetteer, and a tourist beacon in Elite: Dangerous, would rate a portrait... especially when there would be no way of viewing it, short of a history book from Emerald.