The introduction of the LP was certainly a major Mahlerian opportunity. Also, there were performances of Mahler from his day on in various countries, but rather few and far bertween.
The obvious answer as to why the 60s is that 1960 was the centenary year of his birth, and '61 the 50th of his death. That double-whammy was a prompting to performances.
There was also a loosening-up of attitudes in the early 60s. After all, before then, one of Britain's chief critics, Eric Blom, The Times chief critic had recently said,"We don't want Mahler's sort here." And in the 60s, a new generation of critics and of performers began to apear, with more enthusiasm.
Klemperer,Walter,Rosbaud,Mitropoulos,Horenstein,Boult and Scherchen from the old guard were finally given the opportunity, while among the next generation, Bernstein, Leinsdorf, Abravanel, Haitink and Kubelik, and shortly, Barbirolli, were all interested, and the audience was there. among critics, Deryck Cooks, Donald Mitchell, Hans Redlich,William mann and Harold Truscott were pro.
I'm still intrigued th't that Horenstein Albert Hall performance of the 8th in '59 drew a full house to a large hall. Where had the audience sprung from?
As Barry says, the world had to recover from WW2, and The Bomb helped produce an Age of Anxiety, for which Mahler's music seemed on the money.
Once I discovered G.M., around '62 and concerts around '64, I went to as many performances in London as I could manage.
There is much documentation of the growth of performances in the excellent The Mahler Companion, ed. Donald Mitchell. Tho' in it, Mitchell wants to emphasise that in England performances appeared throughout in the 20th century, and th't there was no sudden explosion at the beginning of the 60s. It's certainly easy not to know of the performances up to '59 apart from the recorded ones.
Ivor4