It's hard for guys that aren't used to cutting weight. Heavyweights basically don't have to worry about cutting, so that completely changes the game for them. Jones frame is made for heavyweight and after he adds the bulk he needs, he basically just has to maintain that. He'd have a much easier time I think at HW in terms of training and prep.
I'm also of the mind that HW is the pinnacle in terms of prestige, so it doesn't seem right for the HW champ to move down to face a fighter, even the LHW champ, at a smaller weight class.