Even great maestros and friends of Mahler had favorite symphonies.
Klemperer didn't conduct 1st,3nd, 5th, 8th, 10th(adagio) and I think had conducted M6 only once and decleared the work completelly failure.( More exactly I think he refes to the finale )
Walter didn't make M3,,M6,M7,M8,M10.
We all know that Karajan and Giulini conducted only a handfull.
I consider this fact a very healthy sign. Althought it's the same language, the same world, there are diffrerent neccesities for each work.I beleive to a trully artisic instict of each performer and not to a fashion which wants complete cycles.That said, there are a lot of complete cycles by trully dedicated Mahlerians which are excellent.
Everyone to his likes and dislikes, both artists and musiclovers.