German propaganda. In the 1830s-1850s era there was a lack of German superstar composers unlike the preceding eras. I believe the Germans, considering the influence they had on culture back then, artificially hyped up Schumann to place him amongst the true greats of that era (Liszt, Chopin, Berlioz etc.) Can't have the Polish, Hungarians, French dominating cultural sphere of romantic era music. It explains why Schumann's a part of the standard repertoire, as it certainly isn't because of the quality in the music.
On a semi-unrelated note, i performed Schumann's 2nd sonata last year and my teacher very much adored the performance. Even joked (as I'd like to believe) that I might be better at playing Schumann than Chopin, the latter being my favorite composer. Appalled, I almost fired her on the spot; I've since forgiven her for the transgression. I've made it my mission to never play another Schumann piece again, as to not relive the trauma I experienced during that period (playing Schumann..ugh).
Thal