I think because the reservoir size probably covers most of the population, and the folks with very dry air, or mask leaks can add an inexpensive room humidifier. The reservoir needs a certain area to be able to humidify the air enough in the short transit across the surface of the water. In designing products, I think it is hard to please everyone, all of the time, nor is probably economically wise. There is also a microbial cleanliness issue in that a larger tank might be less likely to be cleaned as often by folks who didn't need the whole volume. That would back up on the device manufacturer in a big way, and if I were them, I would be making the tanks smaller rather than larger to help keep the tanks clean when in use by patients/customers. (Not that I know what ResMed is/was thinking, just guessing)
All the best,
Peter