People have been saying that for quite some time.... but it wasn't really ever meant to be a file system REPLACEMENT for NTFS. It was designed as a subsystem for storage management. Can look it up on wikipedia, and it goes into what it was all about. If i had to guess why it was never really done.... i'd probably have to say because they realised that it would have quite possibly would have caused even more issues with system resources, and bits and pieces of it were probably cannibalized into Vista/Windows 7.
WinFS = Windows Future Storage, not Windows File System.