Ah, but costuming isn't an equation of more elborate = better. Historical costuming--particularly when you also have to costume a large cast of extras--is requires just as much skill and work from the costuming department as ornate fantasy.

Given Dark Crystal won the BAFTA for visual effects, I think this was less an issue of definition than of how the Academy approaches technical awards (from a quick skim since it's inception it looks like only three films are generally nominated) and how in turn that affects film studios willingness to invest the money needed to front a nomination for less financially succesful films, but this is a problem that affects the awards as a whole.