Windows XP is used by 20% of the world's computer-using population. All versions of Mac OS X are somewhere around 7% and I'd imagine Leopard is 1% or less of that. Not worth the resources supporting it. That, and the fact that they're probably trying to force you to upgrade to a new Mac 😛
The problem with iDevices, except for the iPhone and maybe iTouch is that a larger part of the users of those devices are Mac users than, maybe, a random selection of users of computers in general, so I think it's a bit more important to support older devices than before.