To drop down a weight class, Cormier would have to deplete himself even further than he does at LHW by dieting and then cutting water weight. He would be suffering in the process and reducing his strength and physical ability just to fight guys that are naturally smaller and inferior to him.
To fight heavyweight, both Cormier and Jones could just show up at a natural, healthy weight. Less work and suffering is required in the preparation and they'd be fighting at their physical best, with the only challenge being to fight a more difficult opponent in place of the weight cutting process.
So, it comes down to what a person considers more respectable: fighting a better opponent, or depleting yourself in order to fight a lower level opponent. The answer seems obvious to me. Maybe a fitness model or something would respect the latter more but I have more respect for physical abilities than for dieting and cutting weight, and I think most sports fans do as well.
I'm surprised you don't know this stuff already.