Honestly, this is more of a "which" problem than a "why" one.

Is Viper strong because of her walls, which is arguably the best smokes in the game(especially on breeze).

Or is it because of her line-ups, making her the queen of the post-plant meta.

Or could it be that her excellent ability to stall, making her a semi-sentinel(without the nerfs).

Or the fact that she's thicc?