When the tomato industry tires of growing pale hard orbs that taste like water, there's less raw material in the gene pool to use in developing something that actually tastes like a tomato. It's more efficient to grow nothing but soybeans, instead of some soybeans and some wheat and some corn and some vegetables and some apples and over here a duck pond, a pig sty, and a few milkers. In human society, monoculture can have a similar influence. Even overseas, a McDonalds can beat a local diner on price thanks to its low cost of distribution and bulk purchases. Organizations like the World Bank encourage monoculture of crops from millet to eucalyptus so that people will sell crops instead of consuming them, so the country will have exports, so they can get currency, so they can pay back their loans.
Monoculture is anti-sex.
Sex is a risky business, biologically speaking. Two organisms combine genetic material randomly to create new organisms, instead of budding off identical clones. Sex exists to create diversity, and it's worth the risks. If creatures who risked sex to create diversity hadn't survived and reproduced more, we would all be doing something else besides 'it' to reproduce. Even when a species is successful, its genetic line lasts the longest if, instead of producing replicas of the successful model, they go ahead and mix it up with other successful creatures to produce a non-identical next generation.