OS X has a habit of introducing radically new APIs in newer versions of the OS, such as Core Animation in Leopard. There are usually lots of goodies that developers can't help but play with, which then make their apps non-backward compatible. This then cascades to the end-user purchasing OS upgrades from Apple. Now, I may be wrong, but I doubt that there is anything in Core Animation that requires Leopard. Apple could choose to create installable versions of the newer APIs for Tiger (and perhaps Panther). However, they have no financial incentive to do so.
Now, Microsoft also introduces new APIs. The difference is that Microsoft has historically back-ported APIs to previous versions of the OS. For example, WIndows XP shipped with DirectX 8.1. When DirectX 9 came out, Microsoft released it for both Windows 2000 and XP. The same can be said for