Games that only have 1 save slot are balanced accordingly, and have re-spawn points (etc) with possibly some penalty for dying. In ED death is final and requires that you reload your previous save. That in itself strongly implies you will be able to save multiple times.
In a multi-player game, especially a simple action RPG or FPS, there is a reason to separate the character from the world and story. In order to make sure there are no conflicts between different players, the game can just not save the world information between sessions, and save limited quest information (ie keep track of progress so you can not get the same quest reward repeatedly).
Allowing you to re-load in these types of games would guarantee a great deal of cheating (to sell or trade the same equipment multiple times, etc).
D2:ED is a single player game. Given the complexity of the quest system, everything related would have to be saved. If the world can not simply be reset or re-generated, then that has to be saved as well. If you are saving everything anyway, there is no technical reason (that I know of) why you would restrict a single player game to one save.