Who knows why this new material wasn't put into HoME or UT? Only Christopher, actually.
I find one of the reasons behind the publication of the new book to be particularly good. It seems there is the intent that this will form a 'bridge' between LotR and The Sil in terms of narrative and style; how many readers are thoroughly intimidated by The Sil after reading the great narrative of LotR? Well, this new book promises to provide something to bridge that gap. And in addition, it seems that Tolkien himself wanted it to be presented as a full story, so Christopher is fulfilling a desire of his father's.
Bb - we have to be careful though. Remember that Tolkien's children are still very much alive, as are grandchildren who knew and loved him (and who are lovely people who deserve the right to maintain family dignity). Their interests come before anyone's. I believe this is why The Family Album was withdrawn from sale. The Hughes estate also withheld a lot of Plath material and I know they are still hanging onto a lot of it; the couple's children are living, and the whole sorry tale of that relationship is still raw not only in their minds but in the minds of some particularly vicious critics so its a case of protection rather than restriction. The Tolkien estate would run the risk of having every bone picked over were all the material to be made available. We'll just have to accept that our generation won't be the one to see all this stuff, but maybe our great grandchildren will be.