I just checked the Slackware "Current" (unreleased yet, but available for testing) website again to see if the release notes had been updated with the reason to switch from MySQL to MariaDB. Alas, they have not updated that file from the 14.0 release yet. I think you are correct in that there are a lot of applications, including KDE4 I discovered, that use MySQL and Slack must verify they will all work with MariaDB before they release a distro with it.
As another datapoint, I just checked the Slack 14.0 release, the latest release, and MariaDB is not included in the "extra" directory of the distro. That implies that back when 14.0 was released (c. 2012), Slack was not intending to replace MySQL with MariaDB. The "extra" directory usually contains experimental packages that Slack is considering, but they have not yet tested them enough to include them in the normal installation. From what I saw on the "current" website, MariaDB is not going to be in the "extra" directory, but in the installation itself. That change is a big step and a fast decision by Slackware - that is, if it is not changed before the release of "current".
On a personal note, my biggest concern is whether MariaDB will work with the Oracle Java connector I am using to interface with LO Base, or do I have to research all that again to get the correct configuration.
    mysql-connector-java-5.1.18
    jdk/jre-6u37
Yes, it is old, but it works and I don't have to deal with the paranoid Oracle website.
Girvin