I've a theory: Mahler is the new Beethoven. So period instrument orchestras come along in the 1980s, challenging the symphony orchestras' hegemony over Beethoven. By the late 90s, it has become deeply unfashionable, in London at least, to perform Beethoven on anything apart from supposedly 'authentic' instruments. So what do the big orchestras do? They find another Beethoven, but one who lived a hundred years later, giving them a head start of a couple of years before the period bands catch up.
If you've got an orchestra of 80-90, in makes sense to base your repertoire around a composer who actually wrote for those sorts of forces. One who will keep the percussion section busy and show off the volume the brass section are capable of.
It started with Mahler 2, which is definitely the Beethoven 9 of today. Since the 80s it has been the piece to open concert halls and to celebrate orchestras' anniversaries. It has therefore taken over many, although perhaps not all, of the Ninth's many functions. And Mahler 4 is the new Beethoven 6 – the cheery one. Mahler 5 the Eroica, the one that gets played for its slow movement. Mahler 6 the new Beethoven 5, fate and tragedy two sides of the same coin. And Mahler 7 is the new Beethoven 8, the strange one between two uncontested masterpieces that gets wheeled out now and then for curiosity’s sake.
In a way, it's bad luck for Shostakovich. He too wrote effective large-scale substitutes for the Beethoven symphonies. He was fulfilling that role very well until everything became Mahler about five years ago. None of these composers is seriously under-represented in the schedules these days, but lets hope that when the double Mahler anniversary is over we can get a bit more balance among these symphonic masters.