Monocultures are vulnerable to catastrophic phenomena that affect the monoculture. See Irish Potato Famine for one example. Additionally, monocultures tend to

Monocultures are vulnerable to catastrophic phenomena that affect the monoculture. See Irish Potato Famine for one example. Additionally, monocultures tend to deplete soil nutrients, so need land management in the form of fertilizer, crop rotation, or lying fallow. The latter two are often combined. These are (obviously, I hope) not insurmountable.