Why are people (agents) needed "on the ground" in order for the stuxnert virus to work?

Stuxnet virus is a malicious computer worm which was discovered for the first time in 2010. Stuxnet is used to target SCADA [Supervisory Control And Data Acquisition] system. It was the one that is responsible for the serious damage to the Iran nuclear power. 
People are needed on ground for the stuxnet virus to work because, the virus requires a physical implantation in order to work. Such people are usually expert in detecting the most vulnerable spots in a system.