What is the primary geological reason for the far side having fewer maria compared to the near side?

The crust on the far side is significantly thicker

The striking structural asymmetry between the two lunar hemispheres is primarily attributed to the difference in crustal thickness. The crust on the far side of the Moon is substantially thicker than the crust found on the near side. This increased thickness created a much higher barrier, making it substantially more difficult for molten rock originating from the Moon's interior to break through the surface layers. Consequently, while the near side experienced extensive volcanic flooding that created the expansive dark maria, the far side remained largely composed of heavily cratered highlands due to the subsurface molten material being unable to effectively reach and cover the surface.