The FAQ wikipedia put on the subject basically confirms they had admins that were used to Debian. So a derivative made sense for Wikipedia, though they could have been badly burned by jumping on LTS before the LTS process had proven itself (and it still needs a few years before it proves it can deliver as well as RHEL/Centos' nice track record. The last years are the harder in any long term cycle).
It also points yum was less nice than apt two years ago, which may or may not be still true (since yum has been in rapide development since).