It's incorrect to say that you need a saddle 170mm wide because the Brooks feels good to you. The useable area on a Brooks is far less than what the saddle measures side-to-side, due to the relatively large steel/Ti frame that holds up the leather. Most synthetic saddles have more useable area than a Brooks, so you wouldn't need a 170mm wide one.
It also has a lot to do with your bike setup. I have a bike with a relatively upright position and the B17 feels good. On my road bike, with a more aggressive setup, I have a team pro. If I switch these saddles between the bikes, I am not comfortable at all.
The reason saddle fit is so universally difficult is because they are so many factors that seem to affect people to different degrees. Riding style, bike fit, physical condition, and sit bone width are all important. But to say that width is the only factor is oversimplifying a very complicated and often times frustrating problem of cycling.