Why Can't Riot Find A Balance Between Ridiculous Burst and Un-killable Tanks?

Firstly, I totally get that changing the meta keeps things fresh and is valuable from a business perspective to keep players interested. A certain amount of ebb and flow is good for the game and a large part of why it's still so popular. That said- does it really have to constantly jump so drastically between purely tank driven or completely burst dominated metas? They seriously alienate a portion of the playerbase with every swing- either completely invalidating the assassin/burst mage role or making most bruisers/tanks/offtanks nonviable. Speaking personally, I've tried umpteen different ways (various iterations of Guardian, Grasp, or Klepto builds) to resurrect my offtank Bard build that was completely viable not that long ago, but playing him as anything but a scared burst mage with Redemption just doesn't work anymore due to the senseless damage coming from nearly every champion.

Here's a rather intricate puzzle for help with the answer to my question in the title:

** B_ __ _ all base HP and HP growth. N __ all heals, shields and regen.**

Good luck figuring it out. If the Riot devs can't pick up on it I doubt your average league player even has a shot in guessing the answer! :)