To be honest, I don't think Murray has ever liked playing in humid conditions and often struggles when that is the case. He may spend a lot of time training in Miami but actually playing competitive matches in a full blown tournament in those conditions has never been easy for him. He is a pale-skinned Scotsman (have you ever noticed him looking tanned even after he has been in hot places like Miami for a while?). Watch how he struggled in the final of 2013 Miami when it was also exceptionally hot and humid!
Djokovic also dislikes playing in hot, humid conditions and he is tanned and from a much warmer country than Scotland. Imagine how much harder it must be for a pale-skinned Scotsman like Murray?