My best assumption is this, Current gear in PoE has stats that are specific to PoE so once TTK comes out those stats will become irrelevant thus a waste of a tier. ToO does have some gear with great stats, but for some it may be a challenge to get (and from what I've seen, none of the boots grant you more ammo for heavy) which leaves Crota and VoG to have adequate gear for the coming expansion. VoG weapons are still great and some would even say the best in-game even after two expansions. Plus those raids are the only place you can get boots with more heavy. Just another tip, if you don't already, the gloves from Crota may be the best because of the random stat you get when doing damage or a crit to hive you run the chance of generating an orb for teammates, given that in TTK they are still technically Hive if I'm not mistaken.