Signals don't travel on the core of the center conductor, they travel on the skin. The reason for solid-copper is copper is a better conductor than steel which is usually the center for copper-clad. Solid copper results in lower voltage drop and so is preferable in satellite, particularly at longer distances. That is why DishPro is considered to be able to travel over longer distances if installed using solid copper instead of copper-clad.
This is CABL.com posting #211165. Tiny Link: cabl.co/m253