It's all about the geometry and the wheel contact. Consider than an x-8-x steam loco, in reality, only has about 3 wheels solidly on the track at any given moment, maybe 4 at best. And even at that, they are all held in one rigid frame. Even with a little float in the axles, there isn't a lot of solid weight-bearing contact area on many of the wheels.
Compare that to an F unit. You have two swiveling trucks, each of which has only 2 axles and they have a little float to them. You essentially have 8 independent wheels firmly on the track nearly all the time. On any type of real-world layout, it is crucial to have lots of wheel contact points, because the track is not dead level. That F unit can rock and roll and still keep 8 wheels on the track for maximum friction. The steam loco cannot. If our steam locos had fully equalized drivers like the real thing, you would see similar adhesion. But that isn't likely to happen.
