Why is a horizontal rod a full wavelength oriented radially to a broadcast antenna not the best receiving antenna
The waves are not only varying as they propagate radially, but, for a vertical transmit antenna, the EM field will vary vertically as well.  

That's because a vertical antenna has finite height, thus the top and bottom are not radiating at the same phase (due to the speed of light of propagation from the bottom to the top of the antenna).  Thus the radiating EM field will be vertically polarized, and the highest gain will be with an antenna of that same polarization (thus accelerating receive electrons in the same orientation as the accelerating electrons generating the EM waves).

There will be some reception with a radially oriented antenna, that's the basic theory behind a long wire Beverage antenna.  But the gain will be lower because the pair of antennas will be trying to accelerate transmit and receive electrons orthogonally to one another; and the field propagating from the top and bottom of the transmit antenna will be seen out of phase and thus partially cancel at the receive location.