In counterpoint, Gunnerkrigg Court.
I agree that it's more difficult - but a good author thrives in difficult spaces, and it's certainly possible. Thunt ventured into cute romance in the middle of a dungeon, and he did the "leveled up by pooping in the corner" joke in the middle of a dramatic section.
Perhaps an author starts out writing something funny because he doesn't know if it will succeed, and failure in comedy is not so emotionally damaging as failure in drama. The tone of the piece suggests the intrinsic value of the piece to outsiders, whose opinions the author values. Failing or succeeding at a lark is of minor consequence. Failure or success at serious drama is Important. So the author creates a comedy and when it starts doing well he switches to drama.
It may also be that the patron wants comedy because comedy sells, and only later when the author gains some power in the relationship he's able to influence the work and make it more what he wanted to do in the first place. See: M*A*S*H.
You also see this general trend in artistic culture: "I started out doing something lighthearted but now I want to do something serious." Do you start over with a new, serious piece? Or do you warp your comedy into a drama because you already have people paying attention to you there? Regardless of the change virtually ruining the piece?
In sitcoms you start seeing "very special episodes" and eventually it's mostly drama with a couple halfhearted jokes that the live studio audience hardly laughs at. Why does this happen? Are we tired of seeing these characters in comedy and want a change? Should the show remain a comedy because that's what the characters were built for, and if you wanted a drama you should be watching a different show? Why are sitcoms susceptible to this dramatic perversion into soap operas?