It's a compromise between having a geometry that is rigid enough to ensure the bottom and top planes stay aligned while the telescope slews up and down, while h

It's a compromise between having a geometry that is rigid enough to ensure the bottom and top planes stay aligned while the telescope slews up and down, while having a practical shape at the bottom (a square) and avoiding struts going into the light path, which happens if you do an hexapod shape without making the base larger to compensate. Since his initial goal was to make this printable on an ender3 build plate, the smaller square shape was preferable.