I am a classical clarinet player, a Physicist and a programmer. Music theory is ridicously easy in confront to everything STEM I've studied. It isn't even a paragon: the counterintuitiveness of Physics, the abstractions, the rigor of thinking needed was really stressfull to my mind while music theory was... underwhelming to say the least.
I couldn‘t, but I could also not study many other things and not because of what you call difficulty. Quite simply different people are good at some things and less good at others.
Maybe you are better at CS than music and therefore perceive it as easy and the other one as hard.
Have you actually looked into CS deeply? Obviously not. (I‘m not saying this cannot also be true for music, which I don‘t know.)