Biosynthesis is cheaper, faster, and offers better quality control.
Cheaper because less energy inputs and time are needed for biosynthesis, faster life cycle means greater production can be achieved in less time, and quality control means specific compounds can be developed without contamination from other plant-derived contaminants.
That's not to mention that the term "CBD Oil" typically refers to a mixture composed primarily of cannabidiol, but also consisting of several minor cannabinoid constituents like CBC and CBG, which are two canabanoids Nemus indicates are responsible for analgesic affects. These compounds come in trace concentrations in plant material, but can be concentrated and refined more in a biosynthesis process.