not quite what tortuga posted for the full reason. A lot of it has to do with the venturi effect. While it may seem like it's actually harder to breathe, the regulator is forcing air into your mouth at higher than ambient so the pressure differential between your lungs and the regulator is negligible. You can see this if you go vertical upsidedown but rotate so your face is looking up, it will be harder to breathe.
When the diaphragm of the regulator is down, like in normal diving position, the buoyancy of the air is pulling the diaphragm up, making it easier to "crack" and it will just push the air down your throat basically. This is why the regs will free flow if you just pull them out of your mouth without tipping them upsidedown. Now we stop freeflows by turning the regs upsidedown. The lift of the air in the second stage is matched by the weight of the water above it and the free flow stops because the diaphragm can't open. When you breathe with the regulator up, you're having to suck against that when the diaphragm is up which causes resistance when breathing. Depending on how the exhalation diaphragm is you will also get wet breaths as the air escapes out of the exhalation diaphragm.
The post tortuga did was more of why it is difficult to breathe with a really long snorkel as well as work of breathing in a rebreather when there is no positive pressure mechanism
