Why do multivariable control systems require more advanced techniques than single-loop systems?

Multivariable control systems require more advanced techniques than single-loop systems because they involve complex processes where the magnitude of internal variables is dependent upon the values of other related variables within the system.1

In traditional single-loop systems, control is limited to feedback from a single point and correction from a single point. However, in large-scale multivariable processes, these classical control theory techniques are insufficient to manage the interdependencies between variables. Consequently, more sophisticated methods are necessary to develop appropriate control strategies that can account for these interactions and maintain system stability and performance.21