Because the userspace implementations aren't equivalent.
In particular, it is not so easy for them to enforce the following
restriction:
(*) If a non-root user requested the chroot, then setuid/setgid
bits won't have any effect under the new root.
The proposed kernel patch respects (*), but I'm not aware of any
user-level application that ensures (*) is followed.
(Also, there is the small matter that the user-level implementations
are only usable by root, or are setuid root. The latter is only a
minor difference, though, IMHO.)
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