MIT requires that a copy of the license is distributed with the software—which means it is not strictly public-domain equivalent.

CC0 was not intended to be used as a software license, and IRCC includes a clause that the author withholds any patent rights, which which has its own set of concerns.

In practice, either of them would have worked fine, but part of the beauty of public domain code is that you can avoid having to specify a license.

There is no license. And all the contributors are in countries that recognize the concept of public domain. Why should they concern themselves with the niceties of licensing in some countries? How would choosing a different license guarantee a different outcome in those unspecified some countries?