It's a series of radio waves. Radio is made up of photons*, At 5 GHz, the waves are closer together than 2.4 Like this ||||||| instead of like | | | | | | | Therefore there is more of a chance the photons will collide with atoms and stuff in a wall, scatter off, and result in a weaker, less stable signal. I read a bunch of geekspeak on how they supposedly found a way to counter the effect but it gets to a point where you simply can't argue with particle physics.
* Radio, Microwave, UV, Gamma, etc, all photons and it's all light at various "strengths". A very simplified way of looking at it.