Samwise is art director on HOTS. He basically created the whole Blizzard style. He's a god.
I'd imagine HotS also has a dramatically smaller workload and more time to work on content.
It seems more like a labor of love for the artists in HotS, while WoW feels more like artists cranking out art for a product. Not to say WoW art isn't great by any means, but it lacks the emotional sincerity HotS has.
I'm not sure that can be replicated on a large scale in a game like WoW in the way it is present in HotS...