When you have ductwork in the attic your heating/cooling efficiency is hurt by the amount of heat lost through the roof and out into the environment. When the ductwork is all in interior walls, there's no efficiency lost because those losses go into the rooms the system was trying to heat/cool already.

Where I live, we have extreme ranges of temperature (both hot and cold) and almost all ductwork is on the interior for this reason. You'd be spending a fortune in the winter heating an attic when it's getting lost to -17C air temps outside...

It sounds like your attic needs more insulation.