Hows this sound, I have heard that at a microscopic level, a very sharp blades edge has serrations which is what makes it razor sharp. When these serrations get microscopic beads of water stuck in them they rust causing that edge to dull over time without use. So a blade in a humid enviroment will dull without use.
Just a thought, I'd like to hear from those more educated in this area than me, should'nt be hard.