An easy explanation suggests itself in here. Tolkien just never got into writing about the Northern Kingdoms as he had more interesting stuff to think and write about. I mean we know a lot about some places and people (elves, maiar...) but astonishingly little about others. I don't think the prof had any "secret reasons" for posterity to ponder upon why there was such little on some subjects - he just made what he had time for for those things he was more interested himself.
Had he lived for a hundred years we might have the "Histories of the Northern Kingdoms" as well to read?
And what others books could we read then?
"The Maiar: a full account of their birth, relations and deeds"
"The History of the Elves staying in Valinor"
"The fates of the Dwarves of Nogrod"
"Chronicle of the Haradrim"
"The Childhood of Tom Bombadill"
"Traditional Easterling Folklore"
"The secret uncloaked: Wings or Not? The Full History of the Balrogs"
Invent your own!
