The answer to your question lies in clay colloidal interactions based on the cations adsorbed to the colloid.

Magnesium is much smaller atom when hydrated than calcium or potassium, and when present in large amounts causes the colloids to stack together very tightly, creating what we could call ‘colloidal’ compaction.

Calcium, on the other hand, has a larger hydration radius, and moves the colloids apart.

Thus when magnesium is high, it is actually trapped between tightly stacked colloids and not readily available.