why medium format SLR use film backs?
cos the can use the same type of film for different formats, eg 6x6 can use 646 or 66. and switch to different type of film etc
for 35mm SLR, it is not a good idea, Rollai ever make a interchangeable back SLR, but it is too costly and unpopular.
so if a DSLR do have a interchangeable digital back, it will become so expensive that just become an collectors' item.