Microorganisms grow very quickly, creating dense cultures overnight in some cases. Easy to GM, allowing a specific, desired product to be synthesised. Cheap to obtain and can be stocked for long term storage.
A microorganism that grows in a cheap, simple growth media is also highly desirable, reducing costs and making production easier.
Some microorganisms have complicated lifestyles. Therefore, a microorganism that is able to be cultured in a large, industrial fermenter is desirable, reducing complexity.