Monoculture is not nature and is not just not how plants evolved to grow. We can continue to force it as we have been but that obviously isn't working all that well.
Official paper: http://www.nature.com/nature/journal/v406/n6797/abs/406718a0...
Full text alt: http://www.mindfully.org/GE/Rice-Diversity-Yield.htm
Polyculture ag. production's not a bad idea, and it obviously can improve yields under the right conditions, but to be widely adopted it either needs top down advocacy from a larger body (Gov) or be so compelling economically that individuals / companies push for and adopt it at the personal level.
Yields are one thing, but there is also the additional logistical difficulty, and cost, that comes from planting many different crops. Particularly tough when you're talking about some midwest, intensive crop farming, where machinery, layouts, and storage have been designed and optimized for monoculture operations over literally millions of acres. Adapting those operations while maintaining economies of scale is one of the greatest hurdles.
Also, while folks sometimes decry our farming, America isn't as bad as we often make it out to be, and many areas do significantly mix crops. Example:
http://en.wikipedia.org/wiki/File:Crops_Kansas_AST_20010624....
Shows fields of wheat, sorghum, corn, and sections left fallow that are fairly well mixed in Kansas.
Here's the wheat yield per hectare: http://www.earth-policy.org/images/uploads/graphs_tables/upd...
And corn bushels per acre: http://4.bp.blogspot.com/-lvcQDaR56xg/Trfc71LwF3I/AAAAAAAAQK...
Genetic engineering, as well as more traditional agriculture knowledge, has hugely improved productivity.