Why Multiple Patterns in a Large System

Because a large system has different subsystems with different requirements - a single pattern is unlikely to address all of them optimally, and combining patterns allows conflicting NFRs to be handled by using different patterns for different parts of the system

Explanation

Large systems have subsystems with different requirements. For example, the data management layer might use a Repository pattern, the user interface layer might use MVC, and the communication infrastructure might use Client-Server. When there are potential conflicts between non-functional requirements (e.g., performance vs. maintainability), using different architectural patterns for different parts of the system can be a way to reach an acceptable compromise.