One of the main reason for the non adoption of biocontrol agents is the source or authority who sells it. The efficiency of the biocontrol agents are slow rather than instantaneous result.
There are many other reasons and good articles on this topic. Hope our fellow scientist will add further
have a good day
http://dx.doi.org/10.1017/CBO9780511626463.010
Article Barriers to adoption of biological control agents and biolog...
Hi : I think the main idea that the farmer not using the biocontrol agent to control diseases or the insects is the biocontrol agents in slowly in effect of the pest compare with the pesticides, and the farmer preferred to use the faster and more effective way to control the pest.