Mountain bike saddle question...

So what makes a mountain bike saddle any different from a road bike?  I know a road bike saddle is supposed to be comfortable, stiff, lightweight, etc. and I love my Fizik saddle.  But I see there are tons of mountain bike saddles out there and they don't seem to overlap with saddles for roadies.  I get that you might not care as much about shaving grams on an all-mountain or downhill bike, but should the padding come from the shorts and the "give" come from the MTB suspension?