Silver is where the majority of the playerbase is. It's also where people go on "auto pilot" in regards to their skill progression and where it's easy to get stuck for a number of personally inflicted reasons; such as skill plateaus where in your natural talent is no longer sufficient and you have to develop your skills further through practice and conscious effort, people start to think that "time investment translates into a skill increase while ignoring things like consistency in their playing, and finally, most people are apt to blame others before ever daring to turn the critical eye on themselves.
In short, it's the majority of the playerbase and most people don't understand on a fundamental, self aware level that while this is a video game designed to be played for fun, RANKED is a competitive and people have the "just a game approach" in general and wonder why they can't climb.