8-year-old Vista machines are supported by iCloud, but 6-year-old Snow Leopard machines (that cannot upgrade to Lion or above) cannot.
The important thing to remember is that iCloud is not a free service.
The subscription to iCloud comes as part of the price of a
hardware device you purchased from Apple, qualifying hardware being all Apple computing devices capable of running OS X 10.7 or iOS 5 and greater. Windows users can
not create iCloud accounts they can only access ones already created from qualifying apple hardware.
To get what you apparently want Apple would have to create seperate software for OS X 10.6 that, like the Windows software, could only access pre-existing iCloud accounts and not create them. Which would be confusing, and conflict with iCloud's principle revenue generation of selling hardware.
Conversely, Apple weren't going to say "You must run Windows 7 to install this client", and encourage people to buy a new OS from Microsoft.
The current Apple business plan, is that Hardware purchases get you a 'subscription' to OS enabled services. That's why each new OS is now priced at what is basically an administration and accounting fee.
