I think for a few reasons.
Firstly, medicine is a very very competitive course. You generally need a minimum of AAA to be in with a shot. These are very good grades, yet a lot of people won't get in to do medicine. However, these are clearly clever and acedemic people who would do well at uni, thus by having a fifth choice they are likely to still to get to go to uni if they wish.
Also, medicine is a time consuming course to pick applicants for, it involves tests and interviews, and by reducing the number of choices you are decreasing the number of students slightly that apply for each med school, thus reducing the time and cost.