The free space reporting for the Status Bar was removed when the new Libraries were included in the OS. Because of the way the Status Bar reports free space and because it was designed long before the Libraries were conceived, it was seen to be reporting inaccurate information about the free space on the drives. There was a choice of completely recoding the way the Status Bar reported the free space or simply removing that capability. Since there are many other ways to see the free space for a drive, they decided to remove that code.
Another reason is that the Status Bar has always been close to useless when reporting free space, since it is not a realtime component. If the free space on a drive changes, the Status Bar will not reflect the new free space number until there is a refresh of Windows Explorer.
The Status Bar was not completely removed, because it is still useful since it is an easy way to see the number of parent objects in a folder.