Why Global Catalog and Infrastructure master roles cannot be configure in same Domain Controller.

In a forest that contains only a single Active Directory domain, there is no harm in placing both GC and Infrastructure master in same DC, because Infrastructure master does not have any work to do in a single domain environment. But in a forest with multiple and complex domain structure, the infrastructure master should be located on a DC which is not a Global Catalog server. Because the global catalog server holds a partial replica of every object in the forest, the infrastructure master, if placed on a global catalog server, will never update anything, because it does not contain any references to objects that it does not hold.