Why does an upper cutoff frequency exist for rectangular waveguides?

[CPL.Luke]

So I know there is an upper cutoff frequency (at least in the microwave band) for a rectangular waveguide. However for the life of me I can't find why one would exist in the equations, the internet is full of references to it based off of the dimensions of the waveguide, but I'm curious whether or not the various calculators and tables are assuming a certain material is being used to construct the waveguide and thus they calculate the cutoff based on that, or if there is a real EM reaso as to why an upper cutoff exists.

 

[lbrits]

Well, roughly I think it's the point where the longitudinal component goes from being imaginary to real (passing through zero). When it goes real, the wave attenuates. Here's a good explanation: http://en.wikipedia.org/wiki/Cutoff_frequency