The biggest benefit is there is less flex under braking causing less binding. This allows the suspension to work like it was supposed under these more stressful conditions. They tried to go to,much bigger tubes in a conventional layout, but found that while more rigid, were heavier than upside down forks. Hope that helps
What he said. I will add that same theory applies to any bike, especially dirt bikes, not just SMs. Upside down forks also eliminate fork underhang beneath the front axle that conventional forks have. Not as much vulnerablity in the rocks or other nasty off road situations.
Since upside down forks come on most modern dirt bikes, and those are what we use as base models for supermotos, we end up with upside down forks.