Paid mods are problematic because what happens if a game update breaks the API the mod uses? This frequently occurs in many games. Suddenly you've got an unhappy customer, and with money comes liability.

I doubt any game developer would want the burden of being liable to maintain backward compatibility for old mod APIs to support third party mods, but if they take a cut of any money then they ought to be responsible for maintaining that.

Part of the reason KSP has ironically had a modding renaissance is because the community knows that there won't be any updates which could break some of the more ambitious mods being made.