Are you trying to sort the students (so you can compare them, giving higher scores to better students and lower scores to poor students)? If so you will want to include some difficult questions that you expect only a few of the better students will be able to answer correctly. Or do you want to know how many of the students have mastered the content? If your purpose is the latter, you have no need to distribute the scores, so very difficult questions are unnecessary. You will, however, have to decide how many correct answers are needed to demonstrate mastery. Another way to address the "why" question is to identify if this is to be a formative assessment to help you diagnose students' problems and guide future instruction, or a summative measure to determine grades that will be reported to parents.