Because moisture pervades the whole thing and condenses in the cooler portions (as you move through the insulation the temperature drops). So you'd end up with a pool of water in the bottom, or very damp insulation.
For some synthetic insulations that would merely be unpleasant, with down it could easily be fatal.
You want the insulation to be able to breathe. A sleeping bag may have significant moisture in it over the course of a night. Read about transpiration, the quantites of water your body produces are very large. Your sleep insulation system has to be able to handle it...
My personal preference is a dual system. Down inner bag with breathable shell inside and out, covered with an outer bag with a WPB (waterproof breathable) liner and synthetic insulation (FWIW these are very hard to find, I made my own.). The idea is that the moisture stays warm enough to pass through the down bag, and the WPB liner of the synthetic bag and then condenses somewhere in the outer bag if it does at all. The result being drier down protected by waterproof outer bag that can be easily dried and is less susceptible to water problems.
Works well fro me in all sorts of situations.
Damp rainy cool weather (2C and raining) , cold frosty dry weather (-35C and crisp), I use the light WPB lined bag for straight summer use (>15C rain or shine).
Thats how I would do the UQ if you're worried about external moisture coming up from below.
Bookmarks