"Don Giovanni" might be a popular opera on the whole; it doesn't mean every aria in it is popular.I used one of the most popular arias from the opera. Arias used in the quiz were taken from the CD "Mozart arias" which featured Mozart's best music.
In which case, it is perhaps somewhat surprising that people didn't recognize them. Perhaps most of the quiz takers are just not very familiar with Mozart. I am fairly familiar with his better known instrumental music, but not his operas (I dislike opera). But I haven't taken the quiz myself: I have a cap on my internet data transfer, so I can't listen to all that much music online. Wouldn't surprise me in the least if I couldn't tell much of Mozart and Salieri apart, though. Salieri was not a lightweight composer. And of course, the Classical (with a capital C) style did to some extent produce somewhat generic music; it was the taste of the day. Similarly, if you had a quiz featuring fugues by Bach and by some of his contemporaries, people might not easily tell them apart either.
When I watched the film Amadeus, one of the things that struck me that of all the many inserts of opera, the one I actually liked the most was the bit by Salieri! That was of course not at all all the intention of the film makers. ;-)
Anyway, I think this issue of taste in the arts is a complex one. I'm not sure the quizzes test everything one needs to test. Here's something that just occurred to me. I know from personal experience that I can seldom decide at a glance what kind of art/music/literature/films I like. I might watch a film, think it is absolutely superb, and then upon seeing it again it fails to move me. Or I might not think much of it initially, only to find that it sort of lingers with me and eventually grows on me. It happens with music as well. Much of my taste in music is very much an acquired taste.
So it seems to me that in the arts, works often have to marinade a bit before we can decide whether they are great or not. Perhaps a similar thing is at work not just in individuals, but in society as a whole. After a century, some works linger while others disappear, for reasons that are difficult to codify and that are not apparent at first glance. This might be at work in cases like Dickens/Bulwer or Mozart/Salieri, and a quiz that takes a few minutes to do will not test for that effect.
There can be little doubt that hype and publicity plays a big role though. Especially in abstract expressionism. ;-)