For X-Wing it was because all of the Empire's capital ship designs specializing in ant-fighter roles were bigger then they wanted any ship to be in the system.
For Armada it was because all of the Empire's capital ship designs specializing in ant-fighter roles were overspecialized to the point that while they could destroy fighters with ease they had to run like hell if a CR90 showed up becausd their weapons would be lucky to scratch the corvette's paint despite them being two-thirds larger