I play the drums and really enjoy them, but I can't help but notice that compared to other instruments, they're quite easy. There really isn't any theory that's exclusive to them, and you don't have to worry about things like scales or what key the music is in. Drums only really get difficult when it comes to playing fast (16th notes are so hard to maintain). It seems like drums have a lot less that you need to learn to play them well. I'll admit that me finding them easy personally isn't reason enough to believe that they're universally easier than other instruments, but I can't see a reason for them to be harder.