Possibly because enumeration is directionless: it gives flat structures, isomorphic to vectors. , is not a vertical separator, just a separator. Convention considers vectors “vertical”, but they are just vectors, directionless per se. In light of this, I don’t think your table is the right mental model.
[z o, o z] could be given meaning, but space/; is special syntax for a matrix expression and you can’t mix it with anything else (which IMO is a solid design choice). Try [[z o], [o z]].