It's almost like it's hard to use the exposed Anakin head for anything but a (non screen accurate obviously) standing pose.
It's maybe because in the movie you only really see the Humpty Dumpty head in close up, but as a 1/6 laying down display you obviously have to see the whole Vader body and it sort of draws your eyes away from the little pinkish head, which is also inevitably further away from you and partly hidden because of the laying down angle.
The standing display seems to foreground the Anakin head much better - not partly hidden by the laying down angle for starters - and the body rather than being kind of limp as he's laying there can show his power even though he doesn't have the iconic helmet (which I assume most people would have him holding.)
I also think the lightning helmet is tricky too because the clear purple looks slightly odd on the regular (opaque) Vader body - in the movie the see-through/x-ray visual covers his chest and arms as well, which would have made for a cool display but hard for HT to pull off.
