Generally cost but also space. For the finite conjugate, the tube lens is used to demagnify the image as the pi camera sensor is much smaller than the eyepiece:
If we use the same 50mm tube lens with an infinite conjugate lens the sensor will sit 50mm behind the lens, it is a bit smaller for the finite conjugate objective.
More details on the optics implementation, and our plans to simplify the generation code are given in the OpenFlexure Enhancement Proposal OFEP2