The difference is you're hanging off the ground where air is free to travel under you and wick away the precious warmth your body is generating. The same issue does exist in a tent, except it's the ground sucking your heat out of you, and this is why we sleep on foam pads. You can also use a foam pad under you in a hammock but I find it to be uncomfortable having *anything* underneath me in a hammock. So this is why I also no longer use my sleeping bag + pad in a hammock, it was a nightmare gettng in and out of it, constantly squirming about trying to get it in just the right position. I can't even begin to tell you how much more comfortable it is having nothing between you and the hammock, a TQ/UQ setup is expensive but far superior in terms of warmth and comfort.
I was on my second hang last weekend. I simply took my sleeping bag out of the hammock, steped into it, zipped up, and mine has double full length zips, and got into the hammock, lay down and I was out as a candle. No squirming around at all, and warm as toast all night.