People are lazy. Most people here don't know the piece. I haven't even had the time to find out how good or bad all of those pieces are. Really, if you were to ask people entering a concert hall their opinion about Liszt piano concerto in Em or his Eb Posth no one will know anything about it. Depending on the music being played they may not even know Liszt. If its a Mahler symphony some of the people may view non-symphonic music or over-the-top virtuoso music as lesser music.
No one knows those Liszt concerto's so no one cares.
Quality doesn't equal popularity. I am not sure what happened. Liszt probably didn't finish them, they didn't get published, he himself didn't play them. No one else played them etc.
The most famous pianists aren't the best pianists. The most famous music isn't the best music. Therefore music can be totally unknown and very good.
Now considering the classical culture consists mainly of western higher class old rich people cherishing their traditions, you will realise music quality doesn't matter.
If record companies worked together and and tried to promote on of those concerto's who knows what may happen. Maybe all mayor news channels would tell the world about the new discovery: "Liszts third piano concerto". Just like they do with Bach. Then everyone would rave about this new work. Record companies would rush and find pianists willing to record the work. The new progedy pianist, Lang Lang, Yundi Li, or Kissing would record it and everyone would buy it. Then everyone would want to see it in concert. etc etc
Face it - if music is great, it cannot possibly stay unknown throughout all these generations.
What do you think happened with Bach? This is very bluntly untrue. Really, classical music does quite well on this topic because it has so many musicians, scholars, experts, professors, critics etc. But what do you think happens in other styles of music?