Because right now, on live servers, you can run the currently existing dungeons repeatedly and... spend your JP for honor points to buy gear with LFR MSV equivalent item level, making anything lower than that a persistent source of sha crystals. By the time any new dungeons could actually be added though, it would probably be S14 and whatever items dropped in the proposed dungeons would have to be competitive with the honor version of Tyrannical, perhaps even a little better to encourage the use of dungeon drops for PvE over honor gear.
Now, they capped Honor Malevolent at 476 so it wouldn't technically outshine LFR MSV (no weekly wait or faith in virtual dice aside)... so next season will probably be 502? Eh, maybe they'll cap it at 489 so it won't compete with the crap that should have gone to JP this patch, or frankly, should have cost gold and been supplemented with select 496 pieces for valor in the first place, like the way all past rep gear was done.