The reason is again very simple: they are played much much better. The old pieces have been practiced 100 times more and longer, are memorized and well shaped including the smallest details. New music is usually read from the score and has been practiced enough to come through the piece without big accidents. (and then you have to be glad)
If an old masterpiece got played bad - you know that it was played bad (and the piece remains a masterpiece), if an unknown new piece is played bad the audience thinks that the piece is bad - that's life of a contemporary composer.
