Technically that's a limitation of the price point, though. There is no physical reason medium- and large-format DSLR's couldn't be made with higher resolution and sensitivity than film.
Price is a big deal though. You can buy a 4x5 camera and some sheet film for a few hundred bucks online and get started making some amazing photographs right now. To buy a medium format digital back with comparable resolution you're looking at prices rivalling an entry level luxury car.