Q:

Why is it not good to use a waveguide at low frequencies?

Which is the physical phenomenon behind this?

A:

At 300 MHz, the mouth of the waveguide would be of order ~1 meter across. At 30 MHz, ~10 meters across. At 3 MHz, 100 meters. At 300kHz, 1000 meters. At all of these frequencies, coaxial wires are perfectly adequate for conducting signals- and very cheap compared to the cost of making tubular waveguides with the dimensions listed above. 

