In theory monocultures allow for higher yield per acre, but in practice it’s really hard to keep the soil healthy enough to sustain yields over time without adding (eventually) unsustainable amounts of fertiliser.

Crop rotation and leaving land fallow for a year can get you some extra time, but monoculture farming practices are inherently destructive to the soil, and mitigating the damage done takes a long time.

Also worth noting that a huge amount of destructive monoculture farming has nothing whatsoever to do with food production - it’s for bioethanol and such.