Because of Security!
That's the real reason. (And the only real reason, in my opinion -- it's not that hard to make a reader for major file systems, although it's by no means easy; making a writer is the real challenge.)
A program like this bypasses the entire (file) system's security infrastructure, so only an administrator (or someone else who has "Manage Volume" privileges) can actually run it.
So obviously, it wouldn't work in many scenarios -- and I don't think Microsoft (or any other big company) would ever consider making a product like this and then encouraging users to run as administrators, because of the security ramifications.
It would be theoretically possible to make a system which runs in the background and filters out secured data, but in practice it would be a lot of work to get correct and without security holes for production.
By the way I haven't used UltraSearch, but I'd written a very similar program myself a few years ago which I open-sourced just last month! Check it out if you're interested. :)