I was told it is to keep the amount of "media" each guest has as small as possible. All of your MB's and Key cards stay on your
MDE and eventually if you have too many they slow the process of reading it down. When you tap the door it has to search through all of your media and then link to your current resort reservation, the less media to search the better
So if you have a MB then a key card is redundant, and the CM's will try and avoid it if at all possible. If you have no MB and don't want to use your phone the front desk should have no problem issuing a key card.
As someone who had to have my original MDE account closed and open a new one due to too many trips (too much media) I accepted this explanation as it makes sense to me!