Perhaps you hadn't heard of them because nobody is going to put "De Coon dat had de Razor" on his recital. Perhaps you have heard of Felicity Rag because it's not a racist title. A lot of things die into obscurity because they are racist and people don't display those things being that they are offensive to a lot of people. It's not a "natural death"; it is quite deliberate. Until recently, I had never heard of a Popeye the Sailor cartoon called, "You're a Sap, Mr. Jap". It was a WW2 propaganda cartoon. No broadcaster in his right mind is going to play that on the air, because it's racist. Does that mean it "died a natural death" because it was a really poorly made cartoon? No, the production quality is just as good as any other Popeye cartoon. It's just that most thoughtful people in society choose not to present racist material that's going to offend the audience. What's wrong with that?
The problem with Lassus Trombone is that the meaning of the title, and the fact that it is a bad parody of black jazz trombone players, is not immediately obvious if one chooses to be oblivious to it. I was one of those people. I just assumed "Lassus" was somebody's name, and all the glissing was just a cute trick the composer did. Now that I have been educated, I can no longer claim that I don't know. And you can't legitimately argue that nobody knows about this, because obviously some people do. "I only offended 20% of my audience" is a pretty sorry excuse for programming something that's offensive.