I would argue that WTFPL[0] is also fairly close to being a public domain, right there next to CC0. Even FSF recognizes it as a valid (but not a recommended) license[1].
The difference being that large bureaucratic organizations need an army of lawyers to approve the use of open source libraries, and using a common license that is already understood (e.g. CC0) will result in a far smoother experience when the developers in that organization try to use anything to make their miserable lives a touch nicer.
source: I have formerly lead one of the described miserable lives.
[0] http://www.wtfpl.net/
[1] http://www.gnu.org/licenses/license-list.html#WTFPL