Because there's the Electricamp 4x15 like Al Cisneros used to use, which is huge, with the drivers kinda in a diagonal formation, then there's also the Sunn 4x15 with the angled baffles (though with kinda low internal volume, which is probably why it was ported), and then there's also the Hiwatt 4x15 and the Marshall Lemmy style 4x15, which are, I think, just upscaled versions of their bass 4x12 cabs, drivers arranged in a square. Trace Elliot made a 4x15, too, and it was this shape, but ported.
You could also just find a good sealed 2x15 design and just double everything. Maybe even do two chambers (or four, if the 2x15 had each driver in a separate chamber).