This is definitely a big part of it and I think it could be broken down quite a bit deeper than a season-by-season analysis. He is VERY streaky and always has been. On a month-to-month basis his bat will switch from unstoppable to completely dead -and in cases where it doesn’t coincide with the team’s W/L streaks, the hotter times can surely end up being under-appreciated.
A couple other points…he is an EXCELLENT defensive player. When you play first base and you lack superstar-status, that doesn’t get much publicity.
He gets injured a lot. They aren’t lazy injuries stemming from poor conditioning or not taking care of himself, but regardless -it’s frustrating when a player is seemingly always missing time.
Lastly, Giants fans have always had very-very high expectations for Belt, and that goes against him with the detractors. He’s got this great swing, power, discipline, composure, and every year we want him to blossom into one of those top-slugging 30+ HR big-bat first basemen, but never able to quite break through.
Overall I actually find myself in the middle -he isn’t “great,” there’s better guys out there, but he’s solid enough to hold down his spot and he’s been an important piece of the recent dynasty.