Finder was just rewritten from the ground up, from scratch. Heavy investment.
Spaces is about windows management, which has nothing to do with Finder. Same with Expose.
iPhone and iPad deal with files, one way or the other.
For mere mortals, the 'file' as a concept is further up on the pyramid of taxonomy; most people think about their stuff in lower classifications like photos, movies, documents, etc.
When you see a bear, you don't say "Look, an organism!" You say, "Look, a bear!"
Similarly, iPhone and iPad deal with files one way or the other, but the point isn't how it works underneath -- you could have a team rewrite iPhone OS to use Newton-style "soups" and have essentially no change to the user's experience. Files are a convenient abstraction for programmers (and, remember, an abstraction is all they are -- in the end, disks are just a linear stream of bits that the kernel/file system layer "artificially" organize into "files"), and there doesn't seem to be any need to hide a file system from programmers. But users are, in most cases, better off dealing with app-specific document managers.
Spaces and Expose are not "Finder" directly, but they are still concepts trying to deal with the issues around windows, multiple apps, and sharing between apps.