Why do people think it is unreasonable to not support a nearly 10 year old Operating System? Safari 5 is not going to run on OS X 10.1 and in all honesty why would anyone expect it to?
Because 62% [1] of Windows users are still on Windows XP. And these people bought computers; they didn't rent them. It at least feels like Microsoft's duty to herd as many of these people to the modern web stack as possible.
OS X, on the other hand -- virtually everyone is on Snow Leopard or Leopard by now, with a few stragglers on 10.4 Tiger. No one is on 10.1, or even 10.3, anymore. [1]
Part of the difference in the upgrade curves can be attributed to the legendary way Windows bloats and requires beefier hardware with each release (possibly excepting Vista → 7), the fact that Apple only needs to release OSes for its own hardware, the much cheaper and simpler (one version) OS upgrades on the Mac side of the fence, and frankly the anti-consumer DRM/validation tactics of Microsoft that make people fear updates. Apple makes no attempt to stop end users "sharing" or outright pirating OS updates — in part because everyone already bought hardware, but mostly, I suspect, because it's extremely beneficial to the (developer) ecosystem.
Actually, Safari 5 doesn't even run on 10.4, which was the newest operating system until Leopard came out 2.5 years ago. By that measure, Apple's doing worse than Microsoft. (Although, Apple still supports XP in Safari 5.)
