The question, I think, is why "crappy" (relatively) pieces like Fantasie-Impromptu and, I'll add my own, La Campanella get the spotlight more than their related neighbour pieces, i.e. the other Chopin impromptus, which are gorgeous, or the 5 "other" Paganini studies that, although perhaps equally as creative as La Campanella, are more interesting musically than the 3rd etude.
Then there's HR2...
Recently I've spent a lot of time listening to the Hungarian Rhapsodies and I have to say they're all fine pieces of music minus, in my opinion, 2, 14 and 15 (14 is ONLY there because it is much better as the Hungarian Fantasy. It is much too expansive for piano alone--of course Liszt was right on in realising that). So far I've only played no. 6, which is one of my favourite encore pieces, but plan on doing a large amount of other rhapsodies, if not all of them, in the next couple of years. No.'s 1, 4, 5, 6, 7, 9, 10, 11, 12, and 19 are all tops on my list.
As far as the question is concerned, I think it's rather obvious that pieces like HR2, HR6, FI, and LC (which are all, incidentally, so bloody well-known they have their own universal abbreviations) are more famous than there usually more musical and scholarly neighbours because of the extroverted technical flashiness of each. FI and LC in particular are known to be much more difficult than they sound, and that technical display (or at least the imagined difficulty in the audience) tends to impress listeners. Pianists, I think, feel pressured to play the most popular of different sets because 1) they have to compete with others who can play the pieces and 2) they have to play things the audience knows and enjoys, or that have been "traditional" recital pieces for decades.
Topic: Liszt Hungarian Rhapsodies