It states MIT/BSD are special cases, just out of curiousity, what makes them special that they cannot be added?
Because there is no MIT or 1/2/3-clause BSD license. There are hundreds of independent, barely related licenses that are quite similar and, therefore, are considered together as a class of MIT licens*es* (note the plural), 1/2/3-clause BSD licens*es* etc. Despite many of them may be very similar and, in fact, usually they share huge portion of the text, they are formally different agreements.
In the above explanation I do not support any of the sides. Whether classes that share 100% of important content and 99% of formatting content, should be considered similar enough to have a shared entry in Arch’s licenses directory, is a separate decision. I am just explaining.