They are below on winrate because:
Tanks serves a purpose of control and setup for the whole team. Common people rush in without their team to back them up, sometimes their team is there but have no idea what's happening.
(Not only by position as mentioned) This requires a good knowledge of your team's abilities cooldowns, available mana pools. Common pubs Never do that. You, that ETC that pooled the perfect mosh with no follow up, that's you.
Tanks give a false sense of power influenced by their high HP pools and "flash", but it takes them to die under focus only 0.5 second longer. Contributing more to tendency to fall out of sync with their teams.
You will find that in most pubs, passive inexperienced players will play a support or a long range assassin, so they can stay behind the line and make the least mistakes (Some will say girls do that mostly but no one will want to say it), and reckless inexperienced players will play an aggressive assassin or a tank, going to face on the whole world. The result of this, is seen in the winrate.
In the end, tanks are best utilized either as counter initiation (takes experience), or when they are waiting for the perfect setup, not rushing in, the captain makes the call, and they make the initiation with the team willingly knowing to go in. If this can happen in pubs more often, we can all appreciate it. If we'll look at other MOBAs, the brilliancy of balancing the heroes is knowing how they are utilized in high skill levels and low skill levels - Something that Blizzard currently severely lacks, when we're looking at several meta heroes in depth. Certain heroes will fail miserably in low skill, yet triumph in high skill. Yet, this can go vice versa - When balancing comes to