I think part of the reason that Schubert's work isn't as popular as many of the other great composers is that his main body of work isn't in the more widely listened-to genres (Symphonies, Operas, Concertos, for example). Yes the 8th and 9th symphonies are masterpieces, and some of his earlier symphonies certainly deserve more attention than they get, but after that you have to move to chamber music to find his best music and the majority of his output, which many people aren't willing to do. It's a shame, though, because as you say, his piano sonatas, string quartets, piano quintet, and especially his huge catalogue of lieder, are as good as they come.
-Jay