they didnt pick it to be the best tank. the players choose skill combinations and pugs pick the most realized builds to be accepted. it wasnt intended for shadowform to be a permanent invincibility...it just happened with EOTN and im pretty sure they didn't see it coming. they just waited to long to fix it, and the damage was done. after a few months went by, they tried fixing it but just realized a large chunk of their players enjoy the simplicity of shadowform. the more they mess with it, the more players just get sick of the drastic changes and move on to a different game.
one example was the whole ursan thing...when it was realized to be overpowered, they delayed nerfing it for an unknown reason. lots of people quit when it became the staple for parties in like 75% of areas. then, when it was nerfed, even more people quit out of frustration (players who used it religiously).
i think they didnt want to re-nerf shadowform because it would just cause more players to leave because one of their favorite things to do was killed. hopefully this shows they are learning from their mistakes and will at least attempt to preserve class balance in GW2 if possible. even though it ruins the "balance" of pve, it was already mostly ruined in GW1 by that time anyway
