What geographical feature was required before mare volcanism could fill the low elevations?

Massive, deep impact basins

The formation of the maria is entirely dependent on precursor structures created during the Moon's intense early history of bombardment. Massive asteroid and comet impacts fractured the lunar crust profoundly, excavating enormous depressions known as impact basins, often hundreds of kilometers wide. These deep voids acted as the necessary low-lying collection points—the prerequisite geography—where low-viscosity lava could pool and spread out over vast areas over millions of years, forming the flat plains we observe today.