As you lower the IPD you're scrunching the displays and optics closer and closer together and at some point you mechanically run out of room. You'd need to reduce the FOV or add mirrors or something to get the lenses to fit.

There's probably also an element of it not being worth complicating the design to chase users in the long tail of IPDs. (My IPD is about 72, which is slightly outside the range on the other end for this and most other headsets, despite it being less of a mechanical challenge.)