i have a feeling the 6 week wait is there to encourage raaiding.
at this point with how easy it is to jump into keys anytime and push (with rewards up to 20 now) there’s almost 0 incentive to commit to raiding. Putting tier in the raid and only in the vault otherwise is their way of keeping raiders going. if it dropped in keys, it would be a better use of a heroic raid guilds time to do spam keys until every one had tier and then most likely just go into mythic.