The first thing would be to understand what is the meaning of candidate. Candidate implies a person who is appropriate and most likely to get a particular position. MBA studies is basically concerned about management studies and making decisions in administration activities. Hence even though there are theoretical case studies and emulations to be done by MBA students, the key part is the ability of the MBA student or more precisely speaking, the candidate who can make such managerial decisions.
One of the reason to call MBA students as candidates is to eliminate any sort of hierarchy situations in them. We often find that all other courses like engineering and medicine have pre-defined age limits associated with the individuals taking the course. However while doing a MBA, there is no such age limit imposed on students. So there might be an instance where a General Manager of a company might be studying aside an completely new fresher. Hence to solve any hierarchal issues they might be addressed as candidates, to avoid any hierarchical issues.
In the scientific world and pure technological field, there are well defined methods and rules to solve a particular problem. However in the real world, any problem has technological, economic and psychological aspects associated with it. Hence a trade off has to be made between every aspect while arriving at a decision. Hence every different set of decisions, made by different individuals or candidates may be correct, but what is more important is the correct set of solutions pertaining to the existing conditions. Hence no one is an expert in making the right decisions, every candidates views are expert decisions which are right if viewed from the right angle. Hence to generalise the public and eliminate any inferiority or superiority complex we can address MBA students as candidates.