Why Ground-wave (surface-wave) propagation depends on the wave clinging to the boundary between air and the conducting earth. A horizontally polarized electric field runs parallel to that conductive surface, so the ground effectively short-circuits it and the wave dies within a short distance. A vertical electric field is perpendicular to the ground and survives, which is why AM broadcast and 160/80 meter surface-wave work uses vertical antennas over good ground or a radial system.
Watch out Horizontal polarization is the usual choice for HF skywave dipoles and beams, but at ground level those signals are absorbed almost immediately, so they contribute nothing to surface-wave range. Circular and elliptical polarization are terms from satellite and VHF work, not surface-wave propagation.
Ground wave wants the E-field standing up: vertical polarization. Lay it flat and the earth shorts it out.