I can only guess, but I think part of the issue at play is that Sabbath have considered themselves a current band capable of competing in the current music market.
I get the feeling they think that releasing old, archival material is for bands who are not able to be relevant NOW.
Again, this is just a guess.
Maybe now that "The End" has occurred, we'll begin to see the vaults open up, but it will still be a while. These guys have current product recorded THIS year out THIS week!