Why? Because I make mistakes.
I sometimes forget to ward. I sometimes initiate fights we can't win. I sometimes miss my skill shots, and fail to dodge the enemy's. I sometimes pick a champion that doesn't work well with the rest of my team's champions. Sometimes I make a mistake that causes my lane opponent to snowball out of control. I sometimes get fed off my ass and do not make the proper decisions to carry our team's momentum and achieve victory.
Do I get trolls, and afkers? Yeah we all do. But I admit that for the most part, I am bronze because of my own mistakes. After all I can't improve unless I fix what I'm doing wrong.