A couple of other theories:

SQ5 is the only numbered one - other than the single-movement SQ12 -
that is incomplete: the middle movements are presumed lost.  As pianists
sometimes do with sets of the Schubert piano sonatas, perhaps the Kodalys
are only recording the works that are complete.

The old New Grove lists several sets of dances and other works for SQ,
including a very early overture (D8), so there are, as Dave suggests,
enough bits and pieces for at least one more CD.

Richard Pennycuick