You are useless until you get items, which are quite hard to get when people pick champs like
who will bully you out of lane and make it impossible for you to farm.
You meatshield for your Vayne and she gets a quadrakill and says "ez" in /all and givesyou absolutely no credit for her success, while also yelling at you for not peeling when she dies 4v1 because you wouldn't follow her when she went to farm wolves in unwarded enemy jungle. (kind of like support)
You don't do much damage on your own and your playmaking ability is reliant on your team following up on you.
Fact of the matter is bruisers (especially since the juggernaut reworks) will always be more popular than tanks in soloQ, it sounds really selfish but people would rather pick a champion that can potentially carry the game even if it means having no real initiation for late game teamfights, and even more so in TB since they have no idea what kind of team comp/champions they will be facing.