originally nightmare and nighthold was supposed to be the first raid tier, similar to how hihgmaul and BRF were, with nighthold releasing when enough people had progressed in nightmare. this is why there is no tier set in nightmare. you can probably go back to some blizzcon to verify that.
But they it was decided that nighthold wouldn't release for half a year after nightmare, which is about how long a raid tier ussualy lasts. because 7 bosses weren't gonna entertain people for 6 months, and to keep up with their no content draught claims, they said they would do a miniraid in between, which was revealed to by trials of valor around the time they announced 7.1.
While there ofcourse is no hard source on it, little facts hint at it being a decision taken very late in the process, things like no tier set in nightmare, and most of all trials of valor not being in the dungeon journal while nighthold is.