Nope it's because it's the only 4K of memory that BASIC did not use and that wasn't mapped under the system ROM; so it was a common place where to put assembly

Nope it's because it's the only 4K of memory that BASIC did not use and that wasn't mapped under the system ROM; so it was a common place where to put assembly language code to be loaded with a BASIC program, or to be used while the BASIC interpreter was running. Cartridge ROM can appear at various places since they have access to the memory bus but $C000 can't be used as the only place for cartridge ROM.