Two big reasons are:
• There isn’t enough organic fertilizer to meet high-yield farming demands. To even get close to enough would require millions of acres in addition to those that farmers use now.
• Organic fertilizers do not provide a balanced diet for plants.
In livestock manure, for instance, the concentration of nitrogen (N), phosphorus (P) and potassium (K) can vary widely — both in terms of how much of these nutrients is in the manure, as well as the N-P-K ratio.
Applying enough manure to provide adequate nitrogen (N) for corn, over time could likely add 4-5 times as much phosphorus (P) as the crop requires. In short, it’s virtually impossible to closely match fertilizer nutrients with crop needs using only manure. And it’s easy to “naturally” over- or under-fertilize.