That's because Apple provides new API features in new releases of the OS and software developers take advantage of them. It's not that hard to write software that runs on many versions of OS X, but it does limit the features you can take advantage of. I think it's worth not being able to use some apps on my older computers if they provide something worthwhile, so this doesn't seem like a bad thing to me.

Windows does the same thing. DirectX 10 is only available on Vista, for example (Unless of course you hack around this, using something like Alky, but then again, with enough work the same could be done in OS X).

For games sure, but most software in the windows world still lists win98 as a supported platform.

What's the point of running older OSes?

I have a power pc mac so 10.5 is the end of the road. Maybe I don't feel like shelling out 130 to apple for nothing.

i'm a fan of newer hardware with older software for speed reasons.

Newer Mac OS X versions don't tend to be any slower than older versions, though. But I definitely agree with you when it comes to Windows.