Dale, W4OP wrote:
"Could you give an explanation of why this is?"

Joe O. had written:
"A Horizontal Loop will pickup less vertically polarized atmospheric
noise."

ON4UN wrote on page 10-7 of "Low-Band DXing":
"The loop will act as any horizontally polarized antenna over real
ground; its wave angle will depend on the height of the antenna over the
ground."

So, the short answer is "cross-polarization".

Most noise is local and reaches a receiver through ground-wave
propagation. Horizontally polarized waves have zero ground-wave
propagation. Their low-angle reflections are out-of-phase with the
direct wave.

Horzontal antennas are in general quieter than vertical antennas.
Cross-polarization loss has been often reported as about 20 db.

Loops, small in terms of wavelength, tend to have the same volts and
amps induced into all their increments. This helps reduce sensitivity to
overloads from high-power signals at frequencies far below the desired
reception, as might be received on a voltage-probe antenna.

Best regards, Richard Harrison, KB5WZI