To be fair, its mainly just the big two of Marvel and DC but those two companies are so large they eclipse everything else and drag the entire rest of the industry in whatever direction they want to go.

Continuity issues is why I personally have not gotten into comic books. Just finding out where to start is a daunting task. If I want to read Spiderman I have to figure out which runs are must reads and which runs to avoid.

At least with manga I know that I'm mostly getting a homogeneous experience, a single story made by one person with a definite beginning, middle, and end. One Piece is long, with no end in sight but if I wanted to read One Piece I know exactly where to start.