When you have to be accountable to management at university which is now more like a business than anything else it is difficult to get away from this. If a student complains about a low grade, even if you can completely elaborate why it is a “bad design” in a way which another designer could appreciate then the manager which you are explaining it to might not accept it as valid. To avoid being accused of bias or not being able to defend what can be argued by non-designers as subjective is avoided at all cost by lecturers because of this. One of the many reasons design education has got into a rut.