The OS went all 64-bit with Mountain Lion if I recall. That means the hardware must be running the x86-64 architecture, which many older machines are not. Older machines, and versions of OS X were 32-bit.I don't get it? I thought apple required 64 bit applications because their OS requires a 64 bit computer. You mean all this time it's been smoke and mirrors why I can't run Mt Lion on my older computer?
A 64-bit OS can run both 32-bit and 64-bit applications. But a 32-bit system is not capable of using 64-bit ones. It simply comes down to an incompatibility of the processor architecture. It's the same reason that most modern smartphones are unable to run full desktop/laptop operating systems.