Do you mean they fit you really well, or that they go out of their way to provide a big variety of styles, cuts, and sizes so that they potentially fit anyone really well?
No, is not about a large variety of cuts and styles. They became well known because their pants, while generally on the slim side, don't make you look like a hipster in skinny jeans, but they also don't look like parachute pants. In the US, mass market men's pants tend to be really baggy and unflattering (aka "diaper butt") for anyone with a slim or athletic build, and that was the niche Bonobos filled.
In short, their pants just make your butt look good.
In short, their pants just make your butt look good.