If you recall, if you were an iOS developer they made iCloud available for Snow Leopard so you could test it with iOS 5. Besides a few slight bugs it worked just fine. Of course shortly before Lion was released it was no longer available.
Point is, it can be done because it HAS been done. Why the short memories?
I remember that clearly, just as I remember that "Siri" was initially touted to be available for the iPhone 4. That was pulled, and "Siri" became the selling point for the 4S. Some speculated that was due to a new iPhone that was shelved, for whatever reasons, and the "backup" 4S was released. As general consumers wouldn't notice the hardware differences, Apple made "Siri" a consumer point to pimp out the 4S. Lion is in much the same vein, making iCloud exclusive to it forces many to OS X Lion, even though, as you stated, iCloud was initially on the road to SL use.
In the end, it's all about money and image. The main reason Lion is in high use is iCloud integration, followed by high Notebook and iMac sales with Lion. This doesn't necessarily mean it's popular, in fact many Apple/MacRumors/etc threads are filled with complaints and I have a ton of bug reports still open and acknowledged by Apple engineers.
Thankfully, I have SL running on another SSD, with help from the many threads on MacRumors, I finally got Address Book in SL to sync my contacts (after getting the correct info for my iCloud account, the trick that many figured out was removing any images from your contacts as, for some reason, they created duplicates in SL - not in iCloud thankfully). Once that was done, my contacts, mail, bookmarks and calendars are fully syncing in SL 10.6.8.
