Like I said before, I think sometimes we are sitting only on one side of the fence and thinking 'they just held it back so they could deliver it as new content' which I don't think is exactly true. Sometimes we have to at least try and put ourselves in other people's shoes to try and understand decisions and thought processes (crazy, I know, thinking of others rather than ourselves). My logical reasoning would be that they created a brand new gamemode Tier 1, and needed to test it in the water with a large playerbase. They knew that if it went out straight off the bat against Hardcore mode, most people would go to their comfort zone and ignore Tier 1. This is literally one of the main reasons loads of games have rotational playlists so all content is being played at some point which gives them valuable feedback in what players like and dislike - they can then pull it out and rework it where possible or even just leave it out if players hate it that much.
Now they have the unanimous feedback so far on Tier 1 that it hasn't quite worked, we're now seeing HC back. I'm cool with that and don't be surprised if Tier 1 gets a little rework in the future because its something new and they now have some feedback on the mode from more constructive players rather than the large majority moaning about just wanted HC back. I don't think they've done a fanfare about HC returning. They haven't said 'New mode incoming which is HC!'. The tweet above just says, HC is back and thats it.